证明：若N为整数,（2N+1）²-（2N-1）² 定被八整除.

(2N+1)^2-(2N-1)^2
=[(2N+1)+(2N-1)][(2N+1)-(2N-1)]
=4N*2
=8N
所以能被8整除
5x^2-3y^2
=(√5x)^2-(√3y)^2
=(√5x+√3y)(√5x-√3y)
a^2+b^2-c^2+2ab-2bc-2ca
=(a^2+2ab+b^2)-2bc-2ca+c^2
=(a+b)^2-2c(a+b)+c^2
=(a+b-c)^2
=(k+3+2k+2-3k+1)^2
=6^2
=36