6-2=4=2*2
12-6=6=2*3
20-12=8=2*4
30-20=10=2*5
所以通项是:An=2n+2(n-1)+2(n-2)+.2*4+2*3+2*2+2*1
=2(1+2+3+.n-2+n-1+n)
=2(n+1)*n/2
=n(n+1)
6-2=4=2*2
12-6=6=2*3
20-12=8=2*4
30-20=10=2*5
所以通项是:An=2n+2(n-1)+2(n-2)+.2*4+2*3+2*2+2*1
=2(1+2+3+.n-2+n-1+n)
=2(n+1)*n/2
=n(n+1)