5^(n+2)-4^(n+1)-5^(n+1)-4^(n+2)=5^(n+1)(5-1)-4^(n+1)(4+1)
=4×5^(n+1)-5×4^(n+1)=5×4×(5^n-4^n)=20×(5^n-4^n)
所以原式是20的倍数.