## Solve Algebraic Equation

Symbolic Math Toolbox™ offers both symbolic and numeric equation solvers. This topic shows you how to solve an equation symbolically using the symbolic solver `solve`. To compare symbolic and numeric solvers, see Select Numeric or Symbolic Solver.

### Solve an Equation

If `eqn` is an equation, ```solve(eqn, x)``` solves `eqn` for the symbolic variable `x`.

Use the `==` operator to specify the familiar quadratic equation and solve it using `solve`.

```syms a b c x eqn = a*x^2 + b*x + c == 0; solx = solve(eqn, x)```
```solx = -(b + (b^2 - 4*a*c)^(1/2))/(2*a) -(b - (b^2 - 4*a*c)^(1/2))/(2*a)```

`solx` is a symbolic vector containing the two solutions of the quadratic equation. If the input `eqn` is an expression and not an equation, `solve` solves the equation `eqn == 0`.

To solve for a variable other than `x`, specify that variable instead. For example, solve `eqn` for `b`.

`solb = solve(eqn, b)`
```solb = -(a*x^2 + c)/x```

If you do not specify a variable, `solve` uses `symvar` to select the variable to solve for. For example, `solve(eqn)` solves `eqn` for `x`.

### Return the Full Solution to an Equation

`solve` does not automatically return all solutions of an equation. Solve the equation `cos(x) == -sin(x)`. The `solve` function returns one of many solutions.

```syms x solx = solve(cos(x) == -sin(x), x)```
```solx = -pi/4```

To return all solutions along with the parameters in the solution and the conditions on the solution, set the `ReturnConditions` option to `true`. Solve the same equation for the full solution. Provide three output variables: for the solution to `x`, for the parameters in the solution, and for the conditions on the solution.

```syms x [solx, param, cond] = solve(cos(x) == -sin(x), x, 'ReturnConditions', true)```
```solx = pi*k - pi/4 param = k cond = in(k, 'integer')```

`solx` contains the solution for `x`, which is `pi*k - pi/4`. The `param` variable specifies the parameter in the solution, which is `k`. The `cond` variable specifies the condition ```in(k, 'integer')``` on the solution, which means `k` must be an integer. Thus, `solve` returns a periodic solution starting at `pi/4` which repeats at intervals of `pi*k`, where `k` is an integer.

### Work with the Full Solution, Parameters, and Conditions Returned by solve

You can use the solutions, parameters, and conditions returned by `solve` to find solutions within an interval or under additional conditions.

To find values of `x` in the interval `-2*pi<x<2*pi`, solve `solx` for `k` within that interval under the condition `cond`. Assume the condition `cond` using `assume`.

```assume(cond) solk = solve(-2*pi<solx, solx<2*pi, param)```
```solk = -1 0 1 2```

To find values of `x` corresponding to these values of `k`, use `subs` to substitute for `k` in `solx`.

`xvalues = subs(solx, solk)`
```xvalues = -(5*pi)/4 -pi/4 (3*pi)/4 (7*pi)/4```

To convert these symbolic values into numeric values for use in numeric calculations, use `vpa`.

`xvalues = vpa(xvalues)`
```xvalues = -3.9269908169872415480783042290994 -0.78539816339744830961566084581988 2.3561944901923449288469825374596 5.4977871437821381673096259207391```

### Visualize and Plot Solutions Returned by solve

The previous sections used `solve` to solve the equation `cos(x) == -sin(x)`. The solution to this equation can be visualized using plotting functions such as `fplot` and `scatter`.

Plot both sides of equation `cos(x) == -sin(x)`.

```fplot(cos(x)) hold on grid on fplot(-sin(x)) title('Both sides of equation cos(x) = -sin(x)') legend('cos(x)','-sin(x)','Location','best','AutoUpdate','off')``` Calculate the values of the functions at the values of `x`, and superimpose the solutions as points using `scatter`.

`yvalues = cos(xvalues)`
```yvalues =  $\left(\begin{array}{c}-0.70710678118654752440084436210485\\ 0.70710678118654752440084436210485\\ -0.70710678118654752440084436210485\\ 0.70710678118654752440084436210485\end{array}\right)$```
`scatter(xvalues, yvalues)` As expected, the solutions appear at the intersection of the two plots.

### Simplify Complicated Results and Improve Performance

If results look complicated, `solve` is stuck, or if you want to improve performance, see, Troubleshoot Equation Solutions from solve Function.

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