计算（3xy-2x²+1/2y²）（3xy+2x²-1/2y²）

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