原式=(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
=3/10
回答完毕