Probably, he was getting all values as "Infinity
The problem is because of the 1/12 in the denominator.
Both 1 and 12 are integers. Change at least one of them to float to get proper results.
This should be sufficient :
Sigma = (1500.0 * .020)/ ((1.0/12)*(.060)*pow(.040-2.0*a,3));
BTW, this is NOT
what we call an infinite loop.
Your loop is finite
, but the output values are infinite