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)
=8-63/64
=7又1/64
(说明最后加一个1/64又减去一个1/64,利用倒推法得出1/2+1/4+1/8+1/16+1/32+1/64=1-1/64=63/64)
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)
=8-63/64
=7又1/64
(说明最后加一个1/64又减去一个1/64,利用倒推法得出1/2+1/4+1/8+1/16+1/32+1/64=1-1/64=63/64)