1.(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)=2.(1+1/2+1/3+1/4+1/5)*(1/2+1/3+1/4+1/5+1/6)-(1+1/2+1/3+1/4+1/5+1/6)*(1/2+1/3+1/4+1/5)=

问题描述:

1.(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)=
2.(1+1/2+1/3+1/4+1/5)*(1/2+1/3+1/4+1/5+1/6)-(1+1/2+1/3+1/4+1/5+1/6)*(1/2+1/3+1/4+1/5)=

1.(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+1/2+1/3+1/4+1/5-1)-(1+1/2+1/3+1/4+1/5)*(1+1/2+1/3+1/4-1)
=(1+1/2+1/3+1/4)*(1+1/2+1/3+1/4+1/5)-(1+1/2+1/3+1/4)-(1+1/2+1/3+1/4+1/5)*(1+1/2+1/3+1/4)+(1+1/2+1/3+1/4+1/5)
=1/5
2.(1+1/2+1/3+1/4+1/5)*(1+1/2+1/3+1/4+1/5+1/6-1)-(1+1/2+1/3+1/4+1/5+1/6)*(1+1/2+1/3+1/4+1/5-1)
= (1+1/2+1/3+1/4+1/5)*(1+1/2+1/3+1/4+1/5+1/6)-(1+1/2+1/3+1/4+1/5)-(1+1/2+1/3+1/4+1/5+1/6)*(1+1/2+1/3+1/4+1/5)+(1+1/2+1/3+1/4+1/5+1/6)
= 1/6

(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+1/2+1/3+1/4+1/5-1)-(1+1/2+1/3+1/4+1/5)*(1+1/2+1/3+1/4-1)
=(1+1/2+1/3+1/4)*(1+1/2+1/3+1/4+1/5)-(1+1/2+1/3+1/4)-(1+1/2+1/3+1/4+1/5)*(1+1/2+1/3+1/4)+(1+1/2+1/3+1/4+1/5)
=-(1+1/2+1/3+1/4)+(1+1/2+1/3+1/4+1/5)
=1/5
第二题类似的

1.(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)*(1/2+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)*(1/2+1/3+1/4+1/5)-〔1*(1/2+1/3+1/4)+(1/2+1/3+1/4+1/5)*(1/2+1/3+1/4)
=(1+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+1/2+1/3+1/4)-(1/2+1/3+1/4)〕-(1/2+1/3+1/4)
=(1/2+1/3+1/4+1/5)-(1/2+1/3+1/4)
=1/5
2.(1+1/2+1/3+1/4+1/5)*(1/2+1/3+1/4+1/5+1/6)-(1+1/2+1/3+1/4+1/5+1/6)*(1/2+1/3+1/4+1/5)
方法同上
=1/6

1.设a=1/2+1/3+1/4
原式=(1+a)*(a+1/5)-(1+a+1/5)*a
=a^2+6a/5+1/5-6a/5-a^2
=1/5
2.设a=1/2+1/3+1/4+1/5
原式=(1+a)*(a+1/6)-(1+a+1/6)*a
=a^2+7a/7+1/6-7a/6-a^2
=1/6

令a=1/2+1/3+1/4+1/5;b=1/2+1/3+1/4,则原式=(1+b)*a-(1+a)*b=a-b=1/5.第二道同理.自己动脑子吧