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Solving Equations

SymPy's solve() function can be used to solve equations and expressions that contain symbolic math variables.

Equations with one solution

A simple equation that contains one variable like $x-4-2 = 0$ can be solved using the SymPy's solve() function. When only one value is part of the solution, the solution is in the form of a list.

The code section below demonstrates SymPy's solve() function when an expression is defined with symbolic math variables.

In [1]:

from sympy import symbols, solve
x = symbols('x')
expr = x-4-2
sol = solve(expr)
sol

Out[1]:

[6]

To pull the value out of the solution list sol, regular list indexing can be used.

In [2]:

num = sol[0]
num

Out[2]:

The code section below demonstrates SymPy's solve() function when an equation is defined with symbolic math variables.

In [3]:

from sympy import symbols, Eq, solve
y = symbols('y')
eq1 = Eq(y + 3 + 8)
sol = solve(eq1)
sol

Out[3]:

[-11]

Equations with two solutions

Quadratic equations, like $x^2 - 5x + 6 = 0$ , have two solutions. SymPy's solve() function can be used to solve an equation with two solutions. When an equation has two solutions, SymPy's solve() function outputs a list. The elements in the list are the two solutions.

The code section below shows how an equation with two solutions is solved with SymPy's solve() function.

In [4]:

from sympy import symbols, Eq, solve
y = symbols('x')
eq1 = Eq(x*2 -5x + 6)
sol = solve(eq1)
sol

Out[4]:

[2, 3]

If you specify the keyword argument dict=True to SymPy's solve() function, the output is still a list, but inside the list is a dictionary that shows which variable was solved for.

In [5]:

from sympy import symbols, Eq, solve
y = symbols('x')
eq1 = Eq(x*2 -5x + 6)
sol = solve(eq1, dict=True)
sol

Out[5]:

[{x: 2}, {x: 3}]

In [6]:

sol[0]

Out[6]:

{x: 2}

In [7]:

sol[1]

Out[7]:

{x: 3}