How to create a sum?

[Peter M]

Hi,

I want to create a sum with a variable n that goes from 0 to a fixed value.

For example:
the sum of (a*b^(c-n)) where a, b and c are constants and n goes from 0 to the value of c.

Like this:
c
Σ (a*b^(c-n))
n=0

So it would be:
if a = 10 and b = 20 and c = 30

10*20^(30-0) + 10*20^(30-1) + 10*20^(30-2) + ... + 10*20^(30-30)

This can be done with a for sling I would assume but I can't get it to work properly..

import math
a = 10
b = 20
c = 30


for n in range(c):
    result = a*math.pow(b, c-n)

print result

This only gives the result of a*b though.

Any ideas? =)

[Peter M]

never mind.
I solved it =)

import math
a = 10
b = 20
c = 30
n = 0


while n != c:
    result += a*math.pow(b, c-n)
    n += 1
print result

[bvdet]

Why not use the ** operator? The following uses built-in function sum() and a list comprehension.

>>> a,b,c=10,20,30
>>> sum([a*b**(c-n) for n in range(c)])
11302545515789473684210526315789473684200L
>>>