Question

刚入门的,最好能用纸写!就2,3,4题

Answer 1

4.

1（1*2*3）+1（2*3*4）+1（3*4*5）.1(（n(n+1)(n+2))

=[1（1*2）-1(2*3）+1（2*3)-1(3*4）+1（3*4)-1/(4*5）+.+1（n(n+1)-1(n+1)(n+2)]2

=[1（1*2）-1(n+1)(n+2)]/2

=14-1[2(n+1)(n+2)]