1/3乘号4+1/4乘号5+1/5乘号6+1/6乘号7
=(1/3-1/4)+(1/4-1/5)+(1/5-1/6)+(1/6-1/7)
=1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7
=1/3-1/7
=7/21-3/21
=4/21