( 1)n(n+ 1)-(n+2)(n+3)

=n？ +n-n？ -5n-6

=-4n-6

(2){-4n-6} is arithmetic progression.

The first item a_ 1=- 10,

Tolerance d=-4,

(3) n=20 16÷4=504

S=a_ 1×n+d×n(n- 1)/2

=(- 10)×504+(-4)×504×503/2

=504×[- 10+(-2)×503]

=504×(- 10 16)

=-5* 1*2*0*6*4

PS: Add * to prevent misjudgment of Du Niang.