1/5×1/6 +1/6×1/7+1/7×1/8+1/8×1/9+1/9×1/10=(1/5-1/6)+(1/6-1/7)+(1/7-1/8)+(1/8-1/9)+(1/9-1/10)=1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/10=1/5-1/10=2/10-1/10=1/10
