答案是1/5
可以用简便方法算
原式=(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+1/2+1/3+1/4)×1/5－1/5×(1/2+1/3+1/4)
=1/5
答案是1/5
可以用简便方法算
原式=(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+1/2+1/3+1/4)×1/5－1/5×(1/2+1/3+1/4)
=1/5