Hi I have met this function but I didn\'t understand how it came with this answer! 1/2(1+1/2+1/4+...+(1/2)^k-1) = 1/2( 1-(1/2)^k / 1-1/2) Solution sum of terms in a geomentric progression a, ar, ar^2,ar^3,.......ar^n-1 = a(1 - r^m)/(1 - r) applying it here with r=1/2,a =1/2. n =k, you get it.