先化简再计算（x+y)^2-4(x+y)(x-y)+3(x-y)^2并求出x=5,y= -4/5时的值.

=((x+y)-(x-y))*((x+y)-3(x-y))
=2y*(4y-2x)
=4y*(2y-x)
带入得到=4*-0.8*(-1.6-5)=4*0.8*6.6=21.12