(3xy-2x²+2分之一y²）（3xy+2x²-2分之一y²）
=[3xy-(2x²-1/2y²)][3xy+(2x²-1/2y²)]
=9x²y²-(2x²-1/2y)²
=9x²y²-4x^4+2x²y²-1/4y^4
=11x²y²-4x^4-1/4y^4