已知:2x+y=6,x-3y=1,求7y(x-3y)^2-2(3y-x)^2

题目

已知:2x+y=6,x-3y=1,求7y(x-3y)^2-2(3y-x)^2

答案

2x+y=6 (1)
,x-3y=1, (2)
(2)*2-(1)
2x-6y-2x-y=2-6
7y=4
7y(x-3y)^2-2(3y-x)^2
=7y(x-3y)^2-2(x-3y)^2
=(7y-2)(x-3y)^2
=(4-2)*1^2
=2