Equations and systems solver

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## Syntax

`S = solve(eqn,var)`

`S = solve(eqn,var,Name,Value)`

`Y = solve(eqns,vars)`

`Y = solve(eqns,vars,Name,Value)`

`[y1,...,yN] = solve(eqns,vars)`

`[y1,...,yN] = solve(eqns,vars,Name,Value)`

`[y1,...,yN,parameters,conditions]= solve(eqns,vars,'ReturnConditions',true)`

## Description

example

`S = solve(eqn,var)`

solves the equation `eqn`

for the variable `var`

. If you do not specify `var`

, the symvar function determines the variable to solve for. For example, `solve(x + 1 == 2, x)`

solves the equation *x*+1=2 for *x*.

example

`S = solve(eqn,var,Name,Value)`

uses additional options specified by one or more `Name,Value`

pair arguments.

example

`Y = solve(eqns,vars)`

solves the system of equations `eqns`

for the variables `vars`

and returns a structure that contains the solutions. If you do not specify `vars`

, `solve`

uses symvar to find the variables to solve for. In this case, the number of variables that `symvar`

finds is equal to the number of equations `eqns`

.

example

`Y = solve(eqns,vars,Name,Value)`

uses additional options specified by one or more `Name,Value`

pair arguments.

example

`[y1,...,yN] = solve(eqns,vars)`

solves the system of equations `eqns`

for the variables `vars`

. The solutions are assigned to the variables `y1,...,yN`

. If you do not specify the variables, `solve`

uses `symvar`

to find the variables to solve for. In this case, the number of variables that `symvar`

finds is equal to the number of output arguments `N`

.

`[y1,...,yN] = solve(eqns,vars,Name,Value)`

uses additional options specified by one or more `Name,Value`

pair arguments.

example

`[y1,...,yN,parameters,conditions]= solve(eqns,vars,'ReturnConditions',true)`

returns the additional arguments `parameters`

and `conditions`

that specify the parameters in the solution and the conditions on the solution.

## Examples

collapse all

### Solve Quadratic Equation

Open Live Script

Solve the quadratic equation without specifying a variable to solve for. `solve`

chooses `x`

to return the solution.

syms a b c xeqn = a*x^2 + b*x + c == 0

`eqn =$$a\hspace{0.17em}{x}^{2}+b\hspace{0.17em}x+c=0$$`

S = solve(eqn)

S =$$\left(\begin{array}{c}-\frac{b+\sqrt{{b}^{2}-4\hspace{0.17em}a\hspace{0.17em}c}}{2\hspace{0.17em}a}\\ -\frac{b-\sqrt{{b}^{2}-4\hspace{0.17em}a\hspace{0.17em}c}}{2\hspace{0.17em}a}\end{array}\right)$$

Specify the variable to solve for and solve the quadratic equation for `a`

.

Sa = solve(eqn,a)

Sa =$$-\frac{c+b\hspace{0.17em}x}{{x}^{2}}$$

### Solve Polynomial and Return Real Solutions

Open Live Script

Solve a fifth-degree polynomial. It has five solutions.

`syms xeqn = x^5 == 3125;S = solve(eqn,x)`

S =$$\begin{array}{l}\left(\begin{array}{c}5\\ -{\sigma}_{1}-\frac{5}{4}-\frac{5\hspace{0.17em}\sqrt{2}\hspace{0.17em}\sqrt{5-\sqrt{5}}\hspace{0.17em}\mathrm{i}}{4}\\ -{\sigma}_{1}-\frac{5}{4}+\frac{5\hspace{0.17em}\sqrt{2}\hspace{0.17em}\sqrt{5-\sqrt{5}}\hspace{0.17em}\mathrm{i}}{4}\\ {\sigma}_{1}-\frac{5}{4}-\frac{5\hspace{0.17em}\sqrt{2}\hspace{0.17em}\sqrt{\sqrt{5}+5}\hspace{0.17em}\mathrm{i}}{4}\\ {\sigma}_{1}-\frac{5}{4}+\frac{5\hspace{0.17em}\sqrt{2}\hspace{0.17em}\sqrt{\sqrt{5}+5}\hspace{0.17em}\mathrm{i}}{4}\end{array}\right)\\ \\ \mathrm{where}\\ \\ \mathrm{}{\sigma}_{1}=\frac{5\hspace{0.17em}\sqrt{5}}{4}\end{array}$$

Return only real solutions by setting `'Real'`

option to `true`

. The only real solutions of this equation is `5`

.

`S = solve(eqn,x,'Real',true)`

`S =$$5$$`

### Numerically Solve Equations

Open Live Script

When `solve`

cannot symbolically solve an equation, it tries to find a numeric solution using `vpasolve`

. The `vpasolve`

function returns the first solution found.

Try solving the following equation. `solve`

returns a numeric solution because it cannot find a symbolic solution.

`syms xeqn = sin(x) == x^2 - 1;S = solve(eqn,x)`

Warning: Unable to solve symbolically. Returning a numeric solution using <a href="matlab:web(fullfile(docroot, 'symbolic/vpasolve.html'))">vpasolve</a>.

`S =$$-0.63673265080528201088799090383828$$`

Plot the left and the right sides of the equation. Observe that the equation also has a positive solution.

fplot([lhs(eqn) rhs(eqn)], [-2 2])

Find the other solution by directly calling the numeric solver `vpasolve`

and specifying the interval.

V = vpasolve(eqn,x,[0 2])

`V =$$1.4096240040025962492355939705895$$`

### Solve Multivariate Equations and Assign Outputs to Structure

Open Live Script

When solving for multiple variables, it can be more convenient to store the outputs in a structure array than in separate variables. The `solve`

function returns a structure when you specify a single output argument and multiple outputs exist.

Solve a system of equations to return the solutions in a structure array.

syms u veqns = [2*u + v == 0, u - v == 1];S = solve(eqns,[u v])

`S = `*struct with fields:* u: 1/3 v: -2/3

Access the solutions by addressing the elements of the structure.

ans =$$\frac{1}{3}$$

S.v

ans =$$-\frac{2}{3}$$

Using a structure array allows you to conveniently substitute solutions into other expressions.

Use the `subs`

function to substitute the solutions `S`

into other expressions.

expr1 = u^2;e1 = subs(expr1,S)

e1 =$$\frac{1}{9}$$

expr2 = 3*v + u;e2 = subs(expr2,S)

e2 =$$-\frac{5}{3}$$

If `solve`

returns an empty object, then no solutions exist.

eqns = [3*u+2, 3*u+1];S = solve(eqns,u)

S = Empty sym: 0-by-1

### Solve Inequalities

Open Live Script

The `solve`

function can solve inequalities and return solutions that satisfy the inequalities. Solve the following inequalities.

$$x>0$$

$$y>0$$

$${x}^{2}+{y}^{2}+xy<1$$

Set `'ReturnConditions'`

to `true`

to return any parameters in the solution and conditions on the solution.

syms x yeqn1 = x > 0;eqn2 = y > 0;eqn3 = x^2 + y^2 + x*y < 1;eqns = [eqn1 eqn2 eqn3];S = solve(eqns,[x y],'ReturnConditions',true);S.x

ans =$$\frac{\sqrt{u-3\hspace{0.17em}{v}^{2}}}{2}-\frac{v}{2}$$

S.y

`ans =$$v$$`

S.parameters

`ans =$$\left(\begin{array}{cc}u& v\end{array}\right)$$`

S.conditions

`ans =$$4\hspace{0.17em}{v}^{2}<u\wedge u<4\wedge 0<v$$`

The parameters `u`

and `v`

do not exist in MATLAB® workspace and must be accessed using `S.parameters`

.

Check if the values `u = 7/2`

and `v = 1/2`

satisfy the condition using `subs`

and `isAlways`

.

condWithValues = subs(S.conditions, S.parameters, [7/2,1/2]);isAlways(condWithValues)

`ans = `*logical* 1

`isAlways`

returns logical 1 (`true`

) indicating that these values satisfy the condition. Substitute these parameter values into `S.x`

and `S.y`

to find a solution for `x`

and `y`

.

xSol = subs(S.x, S.parameters, [7/2,1/2])

xSol =$$\frac{\sqrt{11}}{4}-\frac{1}{4}$$

ySol = subs(S.y, S.parameters, [7/2,1/2])

ySol =$$\frac{1}{2}$$

### Solve Multivariate Equations and Assign Outputs to Variables

Open Live Script

Solve the system of equations.

$$2{u}^{2}+{v}^{2}=0$$

$$u-v=1$$

When solving for more than one variable, the order in which you specify the variables defines the order in which the solver returns the solutions. Assign the solutions to variables `solv`

and `solu`

by specifying the variables explicitly. The solver returns an array of solutions for each variable.

syms u veqns = [2*u^2 + v^2 == 0, u - v == 1];vars = [v u];[solv, solu] = solve(eqns,vars)

solv =$$\left(\begin{array}{c}-\frac{2}{3}-\frac{\sqrt{2}\hspace{0.17em}\mathrm{i}}{3}\\ -\frac{2}{3}+\frac{\sqrt{2}\hspace{0.17em}\mathrm{i}}{3}\end{array}\right)$$

solu =$$\left(\begin{array}{c}\frac{1}{3}-\frac{\sqrt{2}\hspace{0.17em}\mathrm{i}}{3}\\ \frac{1}{3}+\frac{\sqrt{2}\hspace{0.17em}\mathrm{i}}{3}\end{array}\right)$$

Entries with the same index form the pair of solutions.

solutions = [solv solu]

solutions =$$\left(\begin{array}{cc}-\frac{2}{3}-\frac{\sqrt{2}\hspace{0.17em}\mathrm{i}}{3}& \frac{1}{3}-\frac{\sqrt{2}\hspace{0.17em}\mathrm{i}}{3}\\ -\frac{2}{3}+\frac{\sqrt{2}\hspace{0.17em}\mathrm{i}}{3}& \frac{1}{3}+\frac{\sqrt{2}\hspace{0.17em}\mathrm{i}}{3}\end{array}\right)$$

### Use Parameters and Conditions to Refine Solution

Open Live Script

Return the complete solution of an equation with parameters and conditions of the solution by specifying `'ReturnConditions'`

as `true`

.

Solve the equation $$\mathrm{sin}(x)=0$$. Provide two additional output variables for output arguments `parameters`

and `conditions`

.

syms xeqn = sin(x) == 0;[solx,parameters,conditions] = solve(eqn,x,'ReturnConditions',true)

`solx =$$\pi \hspace{0.17em}k$$`

`parameters =$$k$$`

`conditions =$$k\in \mathbb{Z}$$`

The solution $$\pi k$$ contains the parameter $$k$$, where $$k$$ must be an integer. The variable $$k$$ does not exist in the MATLAB® workspace and must be accessed using `parameters`

.

Restrict the solution to $$0<x<2\pi $$. Find a valid value of $$k$$ for this restriction. Assume the condition, `conditions`

, and use `solve`

to find $$k$$. Substitute the value of $$k$$ found into the solution for $$x$$.

assume(conditions)restriction = [solx > 0, solx < 2*pi];solk = solve(restriction,parameters)

`solk =$$1$$`

valx = subs(solx,parameters,solk)

`valx =$$\pi $$`

Alternatively, determine the solution for $$x$$ by choosing a value of $$k$$. Check if the value chosen satisfies the condition on $$k$$ using `isAlways`

.

Check if $$k=4$$ satisfies the condition on $$k$$.

condk4 = subs(conditions,parameters,4);isAlways(condk4)

`ans = `*logical* 1

`isAlways`

returns logical 1(`true`

), meaning that 4 is a valid value for $$k$$. Substitute $$k$$ with 4 to obtain a solution for $$x$$. Use `vpa`

to obtain a numeric approximation.

valx = subs(solx,parameters,4)

`valx =$$4\hspace{0.17em}\pi $$`

vpa(valx)

`ans =$$12.5664$$`

### Shorten Result with Simplification Rules

Open Live Script

Solve the equation $\mathrm{exp}\left(\mathrm{log}\left(\mathit{x}\right)\mathrm{log}\left(3\mathit{x}\right)\right)=4$.

By default, `solve`

does not apply simplifications that are not valid for all values of $$x$$. In this case, the solver does not assume that $$x$$ is a positive real number, so it does not apply the logarithmic identity $$\mathrm{log}(3x)=\mathrm{log}(3)+\mathrm{log}(x)$$. As a result, `solve`

cannot solve the equation symbolically.

`syms xeqn = exp(log(x)*log(3*x)) == 4;S = solve(eqn,x)`

Warning: Unable to solve symbolically. Returning a numeric solution using <a href="matlab:web(fullfile(docroot, 'symbolic/vpasolve.html'))">vpasolve</a>.

`S =$$-14.009379055223370038369334703094-2.9255310052111119036668717988769\hspace{0.17em}\mathrm{i}$$`

Set `'IgnoreAnalyticConstraints'`

to `true`

to apply simplification rules that might allow `solve`

to find a solution. For details, see Algorithms.

`S = solve(eqn,x,'IgnoreAnalyticConstraints',true)`

S =$$\left(\begin{array}{c}\frac{\sqrt{3}\hspace{0.17em}{\mathrm{e}}^{-\frac{\sqrt{\mathrm{log}\left(256\right)+{\mathrm{log}\left(3\right)}^{2}}}{2}}}{3}\\ \frac{\sqrt{3}\hspace{0.17em}{\mathrm{e}}^{\frac{\sqrt{\mathrm{log}\left(256\right)+{\mathrm{log}\left(3\right)}^{2}}}{2}}}{3}\end{array}\right)$$

`solve`

applies simplifications that allow the solver to find a solution. The mathematical rules applied when performing simplifications are not always valid in general. In this example, the solver applies logarithmic identities with the assumption that $$x$$ is a positive real number. Therefore, the solutions found in this mode should be verified.

### Ignore Assumptions on Variables

Open Live Script

The `sym`

and `syms`

functions let you set assumptions for symbolic variables.

Assume that the variable `x`

is positive.

syms x positive

When you solve an equation for a variable under assumptions, the solver only returns solutions consistent with the assumptions. Solve this equation for `x`

.

eqn = x^2 + 5*x - 6 == 0;S = solve(eqn,x)

`S =$$1$$`

Allow solutions that do not satisfy the assumptions by setting `'IgnoreProperties'`

to `true`

.

`S = solve(eqn,x,'IgnoreProperties',true)`

S =$$\left(\begin{array}{c}-6\\ 1\end{array}\right)$$

For further computations, clear the assumption that you set on the variable `x`

by recreating it using `syms`

.

`syms x`

### Solve Polynomial Equations of High Degree

Open Live Script

When you solve a polynomial equation, the solver might use `root`

to return the solutions. Solve a third-degree polynomial.

syms x aeqn = x^3 + x^2 + a == 0;solve(eqn, x)

ans =$$\left(\begin{array}{c}\mathrm{root}\left({z}^{3}+{z}^{2}+a,z,1\right)\\ \mathrm{root}\left({z}^{3}+{z}^{2}+a,z,2\right)\\ \mathrm{root}\left({z}^{3}+{z}^{2}+a,z,3\right)\end{array}\right)$$

Try to get an explicit solution for such equations by calling the solver with `'MaxDegree'`

. The option specifies the maximum degree of polynomials for which the solver tries to return explicit solutions. The default value is `2`

. Increasing this value, you can get explicit solutions for higher order polynomials.

Solve the same equations for explicit solutions by increasing the value of `'MaxDegree'`

to `3`

.

`S = solve(eqn, x, 'MaxDegree', 3)`

S =$$\begin{array}{l}\left(\begin{array}{c}\frac{1}{9\hspace{0.17em}{\sigma}_{1}}+{\sigma}_{1}-\frac{1}{3}\\ -\frac{1}{18\hspace{0.17em}{\sigma}_{1}}-\frac{{\sigma}_{1}}{2}-\frac{1}{3}-\frac{\sqrt{3}\hspace{0.17em}\left(\frac{1}{9\hspace{0.17em}{\sigma}_{1}}-{\sigma}_{1}\right)\hspace{0.17em}\mathrm{i}}{2}\\ -\frac{1}{18\hspace{0.17em}{\sigma}_{1}}-\frac{{\sigma}_{1}}{2}-\frac{1}{3}+\frac{\sqrt{3}\hspace{0.17em}\left(\frac{1}{9\hspace{0.17em}{\sigma}_{1}}-{\sigma}_{1}\right)\hspace{0.17em}\mathrm{i}}{2}\end{array}\right)\\ \\ \mathrm{where}\\ \\ \mathrm{}{\sigma}_{1}={\left(\sqrt{{\left(\frac{a}{2}+\frac{1}{27}\right)}^{2}-\frac{1}{729}}-\frac{a}{2}-\frac{1}{27}\right)}^{1/3}\end{array}$$

### Return One Solution

Open Live Script

Solve the equation $$\mathrm{sin}(x)+\mathrm{cos}(2x)=1$$.

Instead of returning an infinite set of periodic solutions, the solver picks three solutions that it considers to be the most practical.

`syms xeqn = sin(x) + cos(2*x) == 1;S = solve(eqn,x)`

S =$$\left(\begin{array}{c}0\\ \frac{\pi}{6}\\ \frac{5\hspace{0.17em}\pi}{6}\end{array}\right)$$

Choose only one solution by setting `'PrincipalValue'`

to `true`

.

`S1 = solve(eqn,x,'PrincipalValue',true)`

`S1 =$$0$$`

## Input Arguments

collapse all

`eqn`

— Equation to solve

symbolic expression | symbolic equation

Equation to solve, specified as a symbolic expression or symbolic equation. The relation operator == defines symbolic equations. If `eqn`

is a symbolic expression (without the right side), the solver assumes that the right side is 0, and solves the equation `eqn == 0`

.

`var`

— Variable for which you solve equation

symbolic variable

Variable for which you solve an equation, specified as a symbolic variable. By default, `solve`

uses the variable determined by symvar.

`eqns`

— System of equations

symbolic expressions | symbolic equations

System of equations, specified as symbolic expressions or symbolic equations. If any elements of `eqns`

are symbolic expressions (without the right side), `solve`

equates the element to `0`

.

`vars`

— Variables for which you solve an equation or system of equations

symbolic vector | symbolic matrix

Variables for which you solve an equation or system of equations, specified as a symbolic vector or symbolic matrix. By default, `solve`

uses the variables determined by symvar.

The order in which you specify these variables defines the order in which the solver returns the solutions.

### Name-Value Arguments

**Example: **`'Real',true`

specifies that the solver returns real solutions.

`Real`

— Flag for returning only real solutions

`false`

(default) | `true`

Flag for returning only real solutions, specified as the comma-separated pair consisting of `'Real'`

and one of these values.

`false` | Return all solutions. |

`true` | Return only those solutions for which every subexpression of the original equation represents a real number. This option also assumes that all symbolic parameters of an equation represent real numbers. |

See Solve Polynomial and Return Real Solutions.

`ReturnConditions`

— Flag for returning parameters and conditions

`false`

(default) | `true`

Flag for returning parameters in solution and conditions under which the solution is true, specified as the comma-separated pair consisting of `'ReturnConditions'`

and one of these values.

`false` | Do not return parameterized solutions and the conditions under which the solution holds. The `solve` function replaces parameters with appropriate values. |

`true` | Return the parameters in the solution and the conditions under which the solution holds. For a call with a single output variable, `solve` returns a structure with the fields `parameters` and `conditions` . For multiple output variables, `solve` assigns the parameters and conditions to the last two output variables. This behavior means that the number of output variables must be equal to the number of variables to solve for plus two. |

See Solve Inequalities.

**Example: **`[v1, v2, params, conditions] = solve(sin(x) +y == 0,y^2 == 3,'ReturnConditions',true)`

returns the parameters in `params`

and conditions in `conditions`

.

`IgnoreAnalyticConstraints`

— Simplification rules applied to expressions and equations

`false`

(default) | `true`

Simplification rules applied to expressions and equations, specified as the comma-separated pair consisting of `'IgnoreAnalyticConstraints'`

and one of these values.

`false` | Use strict simplification rules. |

`true` | Apply purely algebraic simplifications to expressions and equations. Setting `IgnoreAnalyticConstraints` to `true` can give you simpler solutions, which could lead to results not generally valid. In other words, this option applies mathematical identities that are convenient, but the results might not hold for all possible values of the variables. In some cases, it also enables `solve` to solve equations and systems that cannot be solved otherwise. For details, see Algorithms. |

See Shorten Result with Simplification Rules.

`IgnoreProperties`

— Flag for returning solutions inconsistent with properties of variables

`false`

(default) | `true`

Flag for returning solutions inconsistent with the properties of variables, specified as the comma-separated pair consisting of `'IgnoreProperties'`

and one of these values.

`false` | Do not include solutions inconsistent with the properties of variables. |

`true` | Include solutions inconsistent with the properties of variables. |

See Ignore Assumptions on Variables.

`MaxDegree`

— Maximum degree of polynomial equations for which solver uses explicit formulas

`2`

(default) | positive integer smaller than 5

Maximum degree of polynomial equations for which solver uses explicit formulas, specified as a positive integer smaller than 5. The solver does not use explicit formulas that involve radicals when solving polynomial equations of a degree larger than the specified value.

See Solve Polynomial Equations of High Degree.

`PrincipalValue`

— Flag for returning one solution

`false`

(default) | `true`

Flag for returning one solution, specified as the comma-separated pair consisting of `'PrincipalValue'`

and one of these values.

`false` | Return all solutions. |

`true` | Return only one solution. If an equation or a system of equations does not have a solution, the solver returns an empty symbolic object. |

See Return One Solution.

## Output Arguments

collapse all

`S`

— Solutions of equation

symbolic array

Solutions of an equation, returned as a symbolic array. The size of a symbolic array corresponds to the number of the solutions.

`Y`

— Solutions of system of equations

structure

Solutions of a system of equations, returned as a structure. The number of fields in the structure correspond to the number of independent variables in a system. If 'ReturnConditions' is set to `true`

, the `solve`

function returns two additional fields that contain the parameters in the solution, and the conditions under which the solution is true.

`y1,...,yN`

— Solutions of system of equations

symbolic variables

Solutions of a system of equations, returned as symbolic variables. The number of output variables or symbolic arrays must be equal to the number of independent variables in a system. If you explicitly specify independent variables vars, then the solver uses the same order to return the solutions. If you do not specify `vars`

, the toolbox sorts independent variables alphabetically, and then assigns the solutions for these variables to the output variables.

`parameters`

— Parameters in solution

vector of generated parameters

Parameters in a solution, returned as a vector of generated parameters. This output argument is only returned if ReturnConditions is `true`

. If a single output argument is provided, `parameters`

is returned as a field of a structure. If multiple output arguments are provided, `parameters`

is returned as the second-to-last output argument. The generated parameters do not appear in the MATLAB^{®} workspace. They must be accessed using `parameters`

.

**Example: **`[solx, params, conditions] = solve(sin(x) == 0, 'ReturnConditions', true)`

returns the parameter `k`

in the argument `params`

.

`conditions`

— Conditions under which solutions are valid

vector of symbolic expressions

Conditions under which solutions are valid, returned as a vector of symbolic expressions. This output argument is only returned if ReturnConditions is `true`

. If a single output argument is provided, `conditions`

is returned as a field of a structure. If multiple output arguments are provided, `conditions`

is returned as the last output argument.

**Example: **`[solx, params, conditions] = solve(sin(x) == 0, 'ReturnConditions', true)`

returns the condition `in(k, 'integer')`

in `conditions`

. The solution in `solx`

is valid only under this condition.

## Tips

If

`solve`

cannot find a solution and ReturnConditions is`false`

, the`solve`

function internally calls the numeric solver`vpasolve`

that tries to find a numeric solution. For polynomial equations and systems without symbolic parameters, the numeric solver returns all solutions. For nonpolynomial equations and systems without symbolic parameters, the numeric solver returns only one solution (if a solution exists).If

`solve`

cannot find a solution and`ReturnConditions`

is`true`

,`solve`

returns an empty solution with a warning. If no solutions exist,`solve`

returns an empty solution without a warning.If the solution contains parameters and

`ReturnConditions`

is`true`

,`solve`

returns the parameters in the solution and the conditions under which the solutions are true. If`ReturnConditions`

is`false`

, the`solve`

function either chooses values of the parameters and returns the corresponding results, or returns parameterized solutions without choosing particular values. In the latter case,`solve`

also issues a warning indicating the values of parameters in the returned solutions.If a parameter does not appear in any condition, it means the parameter can take any complex value.

The output of

`solve`

can contain parameters from the input equations in addition to parameters introduced by`solve`

.Parameters introduced by

`solve`

do not appear in the MATLAB workspace. They must be accessed using the output argument that contains them. Alternatively, to use the parameters in the MATLAB workspace use`syms`

to initialize the parameter. For example, if the parameter is`k`

, use`syms k`

.The variable names

`parameters`

and`conditions`

are not allowed as inputs to`solve`

.To solve differential equations, use the dsolve function.

When solving a system of equations, always assign the result to output arguments. Output arguments let you access the values of the solutions of a system.

`MaxDegree`

only accepts positive integers smaller than 5 because, in general, there are no explicit expressions for the roots of polynomials of degrees higher than 4.The output variables y1,...,yN do not specify the variables for which

`solve`

solves equations or systems. If`y1,...,yN`

are the variables that appear in eqns, then there is no guarantee that`solve(eqns)`

will assign the solutions to`y1,...,yN`

using the correct order. Thus, when you run`[b,a] = solve(eqns)`

, you might get the solutions for`a`

assigned to`b`

and vice versa.To ensure the order of the returned solutions, specify the variables vars. For example, the call

`[b,a] = solve(eqns,b,a)`

assigns the solutions for`a`

to`a`

and the solutions for`b`

to`b`

.

## Algorithms

When you use `IgnoreAnalyticConstraints`

, the solver applies some of these rules to the expressions on both sides of an equation.

log(

*a*) + log(*b*)=log(*a*·*b*) for all values of*a*and*b*. In particular, the following equality is valid for all values of*a*,*b*, and*c*:(

*a*·*b*)^{c}=*a*^{c}·*b*^{c}.log(

*a*^{b})=*b*·log(*a*) for all values of*a*and*b*. In particular, the following equality is valid for all values of*a*,*b*, and*c*:(

*a*^{b})^{c}=*a*^{b·c}.If

*f*and*g*are standard mathematical functions and*f*(*g*(*x*))=*x*for all small positive numbers,*f*(*g*(*x*))=*x*is assumed to be valid for all complex values*x*. In particular:log(

*e*^{x})=*x*asin(sin(

*x*))=*x*, acos(cos(*x*))=*x*, atan(tan(*x*))=*x*asinh(sinh(

*x*))=*x*, acosh(cosh(*x*))=*x*, atanh(tanh(*x*))=*x*W

_{k}(*x*·*e*^{x})=*x*for all branch indices*k*of the Lambert W function.

The solver can multiply both sides of an equation by any expression except

`0`

.The solutions of polynomial equations must be complete.

## Version History

**Introduced before R2006a**

expand all

### R2018a: Support for character vectors has been removed

Support for character vector or string inputs has been removed. Instead, use syms to declare variables and replace inputs such as `solve('2*x == 1','x')`

with `solve(2*x == 1,x)`

.

## See Also

### Functions

- dsolve | isolate | linsolve | root | subs | symvar | vpasolve

### Live Editor Tasks

- Solve Symbolic Equation

### Topics

- Solve Algebraic Equations
- Solve System of Algebraic Equations
- Solve System of Linear Equations
- Solve Algebraic Equation Using Live Editor Task
- Select Numeric or Symbolic Solver
- Troubleshoot Equation Solutions from solve Function

### External Websites

- Beam Bending and Deflection (MathWorks Teaching Resources)

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