a=n^2-1,b=2n,c=n^2+1
a^2=n^4-2n^2+1
b^2=4n^2
c^2=n^4+2n^2+1
a^2+b^2=n^4-2n^2+1+4n^2=n^4+2n^2+1=(n^2+1)^2=c^2
直角三角形