[SOLVED] windows python: quadratic formula

zak100 · 06-06-2018, 07:19 AM

Hi,
I am trying to write the quadratic formula program:

Code:

# program to evaluate roots of quadratic eq using python
import math
a=1; b=1; c=100;
x1 = (-b + math.sqrt(b**2 - 4*a*c))/ (2 * a)
x2 = (-b - math.sqrt(b**2 - 4*a*c))/ (2 * a)
print (x1)
print (x2)

I am getting following errors:
Traceback (most recent call last):
File "quadraticeq.py", line 4, in <module>
x1 = (-b + math.sqrt(b**2 - 4*a*c))/ (2 * a)
ValueError: math domain error

Can somebody please guide me whats the error?

Zulfi.

NevemTeve · 06-06-2018, 07:29 AM

insert two more lines

Code:

# program to evaluate roots of quadratic eq using python
import math
a=1; b=1; c=100;
d=b**2 - 4*a*c
print (d)
x1 = (-b + math.sqrt(b**2 - 4*a*c))/ (2 * a)
x2 = (-b - math.sqrt(b**2 - 4*a*c))/ (2 * a)
print (x1)
print (x2)

Delcaran · 06-06-2018, 07:29 AM

With your values of a, b and c you are trying to calculate sqrt(-399), which of course is not a valid operation.
You should first calculate b**2 - 4*a*c, check if it's positive and only then apply the quadratic formula.

jlinkels · 06-06-2018, 07:33 AM

b^2 - 4ac is negative. So you are trying to take the root from Z- instead of N. Which is the wrong domain.

jlinkels

Delcaran · 06-06-2018, 07:43 AM

Quote:

Originally Posted by jlinkels

So you are trying to take the root from Z- instead of N.

Just to be annoying, the domain of square root function is not only N but R+U{0}, all non-negative real numbers.

jlinkels · 06-06-2018, 07:59 AM

Quote:

Originally Posted by Delcaran

Just to be annoying, the domain of square root function is not only N but R+U{0}, all non-negative real numbers.

No you are not annoying, I am just plain downright stupid. I have been playing around too much with integers the last few months.

But I was close as to why the expression "domain" was used in the error message

jlinkels

pan64 · 06-06-2018, 08:38 AM

or use:

Code:

import cmath as math
....

$python /tmp/a.py
(-0.5+9.98749217772j)
(-0.5-9.98749217772j)
$

zak100 · 06-06-2018, 10:05 PM

Hi,
Thanks all of you. God bless you.

Zulfi.