CODEWITHSUNDEEP
pythonmath
Debus
Debus

Reputation: 23

Why is my python code to solve a quadratic formula doing this?

So I am quite a novice at coding python, so I decided to try my hand at making a quadratic equation solver. After entering all my user inputted variables i get this error: 

Traceback (most recent call last):
  File "C:/Users/insertnamehere/Desktop/quadratic formula solver.py", line 6, in <module>
    root=math.sqrt((b**2)-4*a*c)
ValueError: math domain error

My code is:

import math

a=float(input("A?: "))
b=float(input("B?: "))
c=float(input("C?: "))
root=math.sqrt((b**2)-4*a*c)
x=(-b+root)/2*a
x2=(-b-root)/2*a
print(x)
print(x2)

Any help would be appreciated a lot.

Edit: Forgot to add the actual values i entered.

A?: 6
B?: 1
C?: 2

Upvotes: 2

Views: 1739

Answers (2)

PM 2Ring
PM 2Ring

Reputation: 55469

Python can handle complex numbers, but the math module won't do square roots of negative numbers, for that you need to use cmath.

Demo:

#!/usr/bin/env python

import cmath

a = 1.0
b = 1.0
c = 1.0

root = cmath.sqrt(b * b - 4.0 * a * c)
x1 = (-b + root) / (2.0 * a)
x2 = (-b - root) / (2.0 * a)
print x1
print x2

output

(-0.5+0.866025403784j)
(-0.5-0.866025403784j)

Although i is commonly used by mathematicians as the symbol for the square root of negative one, Python uses j; that convention is common among electronics engineers, since they use i to represent current.

So the above output is equal to (-1 ± sqrt(-3))/2

Upvotes: 3

Marcus M&#252;ller
Marcus M&#252;ller

Reputation: 36346

That is a basic 9th grad math question:

Quadratic equations don't always have real-valued solutions, because you can't take the square root of a negative number. You've entered values for a quadratic equation that simply can't be solved within the real numbers.

EDIT: you posted your values:

so, what is the square root of (1² - 2*6*2) ? You cannot find a solution in the real numbers to that question, and neither can python, that's why it's giving you a math domain error.

Look at it this way: 6x² + x + 2 = 0 really has no real solution. Hint: Look at the graph

Upvotes: 1

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