The last 2 lines of the proof for the formula for integration of x^a, for positive a, not the second half which deals with negative alpha. I don't understand how he gets that sum from (q-1)/(q^(1+a)-1)). And then I don't know how he gets the final result 1/(1+a). The attached file is from page 133 of Courant and John's Intro to analysis and calculus.