Question

先化简再求值(x^2+2分之1xy+2分之1y^2)-(x^2+2分之3xy-2y^2)其中X=-4分之1 Y=2分之1

Answer 1

（1）(x^2+1/2xy+1/2y^2)- (x^2+3/2xy-2y^2)

=x^2+1/2xy+1/2y^2- x^2-3/2xy+2y^2

=-xy+5/2y^2

=-(-1/4)×1/2+5/2×（1/2）^2

=1/8+5/8

=3/4

（2）3y^2-5x^2+3(2x^2-y^2)-2(x^2+2y^2).

=3y^2-5x^2+6x^2-3y^2-2x^2-4y^2

=-x^2-4y^2

=-(-1/2)^2-4*(2)^2

=-1/4-16

=-65/4