How to solve symbolic equations (in Python, using SymPy)

Task

Once we’ve expressed an equation or system of equations using the technique from how to write symbolic equations, we often want the software to solve the equation or system of equations for us.

Related tasks:

How to isolate one variable in an equation

Solution

This answer assumes you have imported SymPy as follows.

from sympy import *                   # load all math functions
init_printing( use_latex='mathjax' )  # use pretty math output

If your equation has just one variable, simply call solve on it. Note that you may get a list of more than one solution.

var( 'x' )
equation = Eq( x**2 + 3*x, -x + 9 )
solve( equation )

$\displaystyle \left[ -2 + \sqrt{13}, \ - \sqrt{13} - 2\right]$

Sometimes you get no solutions, which is shown as a Python empty list.

solve( Eq( x+1, x+2 ) )

$\displaystyle \left[ \right]$

Sometimes the answers include complex numbers.

solve( Eq( x**3, -1 ) )

$\displaystyle \left[ -1, \ \frac{1}{2} - \frac{\sqrt{3} i}{2}, \ \frac{1}{2} + \frac{\sqrt{3} i}{2}\right]$

To restrict the solution to the real numbers, use solveset instead, and specify the real numbers as the domain.

solveset( Eq( x**3, -1 ), domain=S.Reals )

$\displaystyle \left\{-1\right\}$

(If solveset gives no solution, it shows it as the empty set symbol, $\emptyset$.)

You can solve systems of equations by calling solve on them.

var( 'x y' )
system = [
    Eq( x + 2*y, 1 ),
    Eq( x - 9*y, 5 )
]
solve( system )

$\displaystyle \left\{ x : \frac{19}{11}, \ y : - \frac{4}{11}\right\}$

Content last modified on 24 July 2023.

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Contributed by Nathan Carter (ncarter@bentley.edu)