20-5/2-9/4-17/8-33/16-65/32-129/64==20-(2+1/2)-(2+1/4)+(2+1/8)-(2+1/16)-(2+1/32)-(2+1/64)=20-2×6-(1/2+1/4+1/8+1/16+1/32+1/64)=8-(32+16+8+4+2+1)/64=8-63/64=7又1/64