把后一个分数换成两个分数相减
原式=1/128+1/64+1/32+1/16+1/8+1/4=(1+2+4+8+16+32)x1/128=63/128
=1/4+1/4-1/8+1/8-4 1/6+1/4-1/32+1/32+1/64+1/64-1/128=1/2-1/128=63/128