1.计算：2的-1次方+2的-2次方+2的-3次方+...+2的-2013次方.
= [ 1/2 * (1-1/2^2013)] / (1-1/2)
= 1- 1 /2^2013
2
9的n次方×27的n-1次方÷3的3n+1次方=81
3^(2n) * 3^(3n-3) ÷ 3^(3n+1) = 81
3^(2n+3n-3 -3n-1) =81
3^(2n-4) =81
2n-4 = 4
2n =8
n=4
n^(-2)
= 4^(-2)
= 1/4²
=1/16
肯定对
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