3^(N+3)-4^(N+1)+3^(N+1)-2^(2N)
=27*3^N-4*4^N+3*3^N-4^N
=30*3^N-5*4^N
=30*3^N-20*4^(N-1)
=10*(3^(N+1)-2*4^(N-1))
因此能被10整除