Question

已知x-2y=3,求1/2x²-2xy+2y²

还有一题、

已知 x²+y²=20，x²+xy-2y²=2（x+2y），求x+y的值

Answer 1

1/2x²-2xy+2y²
=1/2x²-xy-xy+2y²
=1/2x(x-2y)-y(x-2y)
=3/2x-3y
=3/2(x-2y)
=9/2

Answer 2

原式=（x-2y)(1/2x-y)=1/2(x-2y)(x-2y)=9/2