一道六年级的简算题

=1/5
把1/2+1/3+1/4设为一个植X
一切就简单了

(1+1/2+1/3+1/4)*(1/2+1/3+1/4+1/5)-(1+1/2+1/3+1/4+1/5)*(1/2+1/3+1/4)
=(1/2+1/3+1/4+1/5)+(1/2+1/3+1/4)*(1/2+1/3+1/4+1/5)-(1/2+1/3+1/4)-(1/2+1/3+1/4+1/5)*(1/2+1/3+1/4)(把两个括号里面的1乘出来，有两项可以消除）
=(1/2+1/3+1/4+1/5)-(1/2+1/3+1/4)
=1/5

(1+1/2+1/3+1/4)*(1/2+1/3+1/4+1/5)-(1+1/2+1/3+1/4+1/5)*(1/2+1/3+1/4)
=(12+6+4+3)/12*(30+20+15+12)/60-(60+30+20+15+12)/60*(6+4+3)/12
=(25*77)/720-(137*13)/720
=144/720
=1/5