quadratic equation for python NEW

JAPANBOY · 06-26-2011, 11:44 AM

few days ago i posted something about the quadratic equation on how to write it in python and i wrote something like this

A=int(input("enter a number for A :"))
B=int(input("enter a number for B:"))
C=int(input("enter a number for C:"))

then i wrote

quadratic_equation=a*x**2+b*x+c

so the question im asking is how do i make python know that im making X a variable because i get this

Traceback (most recent call last):
File "<pyshell#7>", line 1, in <module>
QE=a*x**2+b*x+c
NameError: name 'x' is not definedTraceback (most recent call last):
File "<pyshell#7>", line 1, in <module>
QE=a*x**2+b*x+c
NameError: name 'x' is not defined

thank you !!!

theNbomr · 06-26-2011, 12:18 PM

Code:

NameError: name 'x' is not defined

What happens when you define 'x'? I think it would correctly assign the result of the expression to the variable 'quadratic_equation'.

--- rod.

firstfire · 06-26-2011, 02:40 PM

Hi.
If you want to solve quadratic equation with given coefficients (numbers) then you should find (calculate) numbers x such that a*x**2+b*x+c == 0. That means you should define a function which performs such calculation, e.g.:

Code:

from cmath import sqrt  # numerical math functions in complex domain

# solves equation
# a*x^2 + b*x + c = 0
# returns: [D, x1, x2],
# where D (discriminant) could be:
#       positive - two different real roots;
#       0 - x1=x2, real;
#       negative - both roots are complex.    
def solve_quadratic(a,b,c):
        D = b**2 - 4*a*c
        x1 = (-b + sqrt(D) )/2.0/a
        x2 = (-b - sqrt(D) )/2.0/a
        return [D, x1, x2]

# test
print solve_quadratic(1,4,1)

If you want to save a quadratic expression to a variable symbolically then you should probably look at `sympy', `sage', `maxima' (not in python) or similar software for symbolic mathematics.

Finally, if you want to be able to compute quadratic expression a*x**2+b*x+c with different values of `x' you may define a function, for example:

Code:

# values of a,b and c are global
quadratic_equation = lambda(x): a*x**2+b*x+c
# evaluate at x=5
print quadratic_equation(5)

graemef · 06-26-2011, 11:59 PM

I think that what you want is to make quadratic_equation a function with variable x, something like:

Code:

def quadratic_equation(x) return a*x**2+b*x+c

where a, b and c are globals and you pass in to quadratic_equation the value of x that you are interested in:

Code:

<<<quadratic_equation(0)
<<<quadratic_equation(1)
<<<quadratic_equation(2)