(1＋1/2)(1+1/4)(1+1/16)(1+1/256)…(1+1/1的2n-2次方)1+1/2的2n次方）
先乘以(1-1/2),整个式子再除以(1-1/2)
=(1-1/2)(1＋1/2)(1+1/4)(1+1/16)…(1+1/2的2n+2次方)(1+1/2的2n次方)/(1/2)
=(1-1/4)(1+1/4)(1+1/16)…(1+1/2的2n+2次方)(1+1/2的2n次方)*2
=(1-16)*(1+1/16)*（1+1/256)……(1+1/2的2n+2次方)(1+1/2的2n次方)*2
=(1-1/2的2n次方)(1+1/2的2n次方)*2
=(1-1/2的4n次方)*2
=2-1/2的(4n-1)次方