2的n次方+2的n-1次方+·····+2+1
问题描述：
2的n次方+2的n-1次方+·····+2+1
答
令s=2^n+2^(n-1)+.+2+1
2s=2^(n+1)+2^n+.+2^2+2^1
2s-s=2^(n+1)-1
s=2^(n+1)-1