Exercise: Prove this ÒcubingÓ program
int n, i, sum, fd, sd;
// assert n > 0
sum = 0;
fd = 1;
sd = 6;
for( i = 0; i < n; i++ )
{
sum = sum + fd;
fd = fd + sd;
sd = sd + 6;
}
return s1;
}// assert s1 == n^3