(1/8+1/9+1/10+1/11)*(1/9+1/10+1/11+1/12)-(1/8+1/9+1/10+1/11+1/12)*(1/9+1/10+1/11)等于?要容易看懂的急
问题描述:
(1/8+1/9+1/10+1/11)*(1/9+1/10+1/11+1/12)-(1/8+1/9+1/10+1/11+1/12)*(1/9+1/10+1/11)等于?
要容易看懂的
急
答
设1/9+1/10+1/11=a
∴(1/8+1/9+1/10+1/11)*(1/9+1/10+1/11+1/12)-(1/8+1/9+1/10+1/11+1/12)*(1/9+1/10+1/11)
=(1/8+a)(a+1/12)-(1/8+a+1/12)a
=1/96+a/8+a/12+a²-a/8-a²-a/12
=1/96