(3x-4y)^2-(3x+y)^2
问题描述：
(3x-4y)^2-(3x+y)^2
(x+1/2y-3)(x-1/2y+3)
已知x+y=5,X^2-Y^2=15,求x-y的值
是 x加二分之一y减去3
你们的答案我看不懂~
好多不一样的%……
答
求什么?
(3x-4y)^2-(3x+y)^2
=[(3x-4y)+(3x+y)][(3x-4y)-(3x+y)]
=(6x-3y)(-5y)
=-15y(2x-y)
(x+1/2y-3)(x-1/2y+3)
=[x+(1/2y-3)][x-(1/2y-3)]
=x^2 - (1/2y-3)^2
=x^2 - 1/4y^2 + 3y - 9
x^2 - y^2 = (x+y)(x-y)=5(x-y)=15
x-y=3