解
(x+3)(x-3)(x²-9)
=(x²-9)(x²-9)
=(x²-9)²
=x^4-18x²+81
(2x+y)²-(2x+3y)(2x-3y)
=4x²+4xy+y²-(4x²-9y²)
=(4x²-4x²)+4xy+(y²+9y²)
=4xy+10y²