1/（a2-a1)+1/(a3-a2)+…+1/a(n+1)-an
=1/2+1/2^2+…+1/2^n
=1/2+1/2^2+…+1/2^n+1/2^n-1/2^n
=1-1/2^n1/a(n+1)-an为什么成了1/2^n+1/2^n-1/2^n？1/2^n-1/2^n不是等于零吗？1/2^n-1/2^n是怎么来的？+0