原式=1-（1-1/2)-(1/2-1/4)-(1/4-1/8)-……-(1/32-1/64)-(1/64-1/128)-(1/128-1/256)
=1-1+1/2-1/2+1/4-1/4+1/8-……-1/32+1/64-1/64+1/128-1/128+1/256
=1/256