Question

一列数依次为-1,8/5,- 15/7,8/3,- 35/11,48/13,.,则这个数列的第十项是（　　　）.

Answer 1

-1,8/5,- 15/7,8/3,- 35/11,48/13

-3/3,8/5,- 15/7,24/9,- 35/11,48/13

an=(-1)^n*[(n+1)^2-1]/(2n+1)

a10=(-1)^10*[(10+1)^2-1]/(2*10+1)

=(120-1)/(20+1)

=120/21

=40/7