Question

数列1又1/2,2又1/4,3又1/8,4又1/16,.的前n项和为多少?

Answer 1

1/2+1/4+1/8+……1/2N=(1-1/2)+(1/2-1/4)+……+（1/N-1/2N)

=1-1/2N

1+2+3+4+……+N=N(N+1)/2

所以原式=(1-1/2N)+(N(N+1)/2)