1.(2n+1)^2-25
=4n^2+4n+1-25
=4n^2+4n-24
4n^2,4n,-24三部分都能被4整除,所以(2n+1)^2-25能被四整除
2.（2n+1）^2-(2n-1)^2
=(2n+1+2n-1)*(2n+1-2n+1)
=4n*2
=8n
所以能被8整除,是8的倍数