Question

一乘二分之一加二乘三分之一加三乘四分之一加......n乘n加一分之一

急，快速
19号14：30之前回答，要不然关闭问题。

Answer 1

1/n(n+1)
=[(n+1)-n]/n(n+1)
=(n+1)/n(n+1)-n/n(n+1)
=1/n-1/(n+1)
所以原式=1-1/2+1/2-1/3+……+1/n-1/(n+1)
=1-1/(n+1)
=n/(n+1)

Answer 2

倾斜角为π/4
k=tanπ/4=1
则-(2a2-7a+3)/(a2-9)=1
且a2-9≠0
-(2a-1)(a-3)/(a+3)(a-3)=1
-(2a-1)/(a+3)=1
1-2a=a+3
a=-2/3