ProjectEuler#44 Pentagon numbers

#Pentagonal numbers are generated by the formula, Pn=n(3n-1)/2.
#The first ten pentagonal numbers are:

#1, 5, 12, 22, 35, 51, 70, 92, 117, 145, ...

#It can be seen that P4 + P7 = 22 + 70 = 92 = P8. However,
#their difference, 70 - 22 = 48, is not pentagonal.

#Find the pair of pentagonal numbers, Pj and Pk, for which their sum and
#difference are pentagonal and D = |Pk - Pj| is minimised;
#what is the value of D?

import time

def pentagonal(n):
    if n >= 1:
        return (n * (3 * n - 1) )/2

def isPentagonal(n):
    if (((24*n+1)**0.5)+1)%6 == 0:
        return True
    else:
        return False

start = time.time()

pentagonals = []
n = 0
flag = True
while flag:
    n += 1
    newPentagonal = pentagonal(n)
    pentagonals.append(newPentagonal)
    for i in pentagonals:
        if isPentagonal(newPentagonal - i)\
           and isPentagonal(newPentagonal + i):
            print newPentagonal - i
            flag = False

end = time.time()
print end - start