当x=0.5 y=2时求(3xy-2x^2+0.5y^2)(3xy+2x^2-0.5y^2)的值
问题描述:
当x=0.5 y=2时求(3xy-2x^2+0.5y^2)(3xy+2x^2-0.5y^2)的值
答
(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