Given the integral e^x dx between a & b exists, evaluate it using the formula 1+r+r^2...r^n = 1 - r^(n+1)/(1-r), where r doesn't = 1.

I tried, using (delta)x = (b-a)/n and xi = a + i(delta)x, but when I get to taking the limit I get stuck. I'm not even sure if that's what I'm suppose to do... but I'm not sure of any other way to go about it. Please help!