（3xy-2x^2+0.5y^2）（3xy+2x^2-0.5y^2）
= [3xy-(2x^2-0.5y^2）][3xy+(2x^2-0.5y^2）]
= (3xy)^2 - (2x^2-0.5y^2）^2
= 9 - (0.5 - 2)^2
= 9 - 2.25
= 6.75