(a^n b^4)³·a^5 b^2n-9 -(a^n+1 b)³·（ab^n）²
=a^3n b^12·a^5 b^2n-9 -a^(3n+3) b³·a²b^2n
=a^(3n+5) b^(2n+3) -a^(3n+5) b^(2n+3)
=0