（3n+1)(3n-1)-(3-n)(3+n)
=9n^2-1-(9-n^2)
=9n^2-1-9+n^2
=10n^2-10
=10(n^2-1)是10的倍数.
n=1时,（3n+1)(3n-1)-(3-n)(3+n)=0,是10的倍数.0是10的倍数？？0当然是10的倍数。10的0倍