(1+1/2)*(1+1/4)*(1+1/8)*.*(1+1/2的n次方)
=2*1/2*(1+1/2)*(1+1/4)*(1+1/8)*.*(1+1/2的n次方)
=2*(1-1/2)(1+1/2)*(1+1/4)*(1+1/8)*.*(1+1/2的n次方)
=2*(1-1/4)*(1+1/4)*(1+1/8)*.*(1+1/2的n次方)
=2*(1-1/8)*(1+1/8)*.*(1+1/2的n次方)
=2*(1-1/2的n次方)*.*(1+1/2的n次方)
=2*(1-1/2的n+1次方)
=2-1/2的n次方