Find the sum of the first 15 terms of the series 4+16+64+256 + ...

A sum of an geometric sequence is a (1-r^n) / (1-r) where n=15 in this problem and r=4 since each term is being multiplied by 4 to get the next term. a also equals 4 since that is the first number. So now you have 4 (1-4^15) / (1-4) which you can type into your calculator. Be sure to use the parenthesis so the calculator will know what is in the numerator and what is in the denominator,