3^2-1^2 =8=8*1
5^2-3^2=16=8*2
7^2-5^2=24=8*3
9^2-7^2=32=8*4
.
n^2-(n-2)^2
=4n-4
=4(n-1)
=8*(n-1)/2 (n为奇数)