Question

当x=2,y=2/5时,(x-y)(x-2y)-1/2(2x-3y)(x+2y)的值为（ ）

四个选项：a.-2/3 b.2/3 c.-2 d.2
我的财富值不多了，望请见谅，谢谢！

Answer 1

原式=x²-3xy+2y²-x²-1/2xy+3y²
=-7/2xy+5y²
=-14/5+4/5
=-2
选c

Answer 2

x=2,y=2/5
(x-y)(x-2y)-1/2(2x-3y)(x+2y)
=(x-y)(x-2y)-1/2(2x-2y-y)(x+2y)
=(x-y)(x-2y)-(x-y)(x+2y) +1/2 y(x+2y)
=(x-y)(x-2y-x-2y) +1/2 y(x+2y)
=-4y(x-y) +1/2 y(x+2y)
=-4xy + 4y^2 +1/2xy+y^2
=-7/2xy+5y^2
=-7/2*2*2/5 + 5*(2/5)^2
=-14/5+4/5
=-2