lim（1-1/2n）.(1+1/2+1/4+.+1/2^n）
lim（1+1/2+1/4+…….+1/2^n）（1-1/2n)
lim[(1-1/2^n+1)/(1-1/2)][1-1/2n]
lim[2-1/2^n][1-1/2n]
=lim[2-1/2^n]lim[1-1/2n]
=2*1
=2