3^(n+2) - 2^(n+2) + 3^n -2^n
=9*3^n+3^n-4*2^n-2^n
=10*3^n-5*2^n
=10*3^n-10*2^(n-1)
=10*[3^n-2^(n-1)]
所以对于任意正整数n,3^(n+2) - 2^(n+2) + 3^n -2^n能被10整除