Question

已知 x y = 1 3 ，求 3x x+y - x 2 - y 2 x-y • 3y x 2 +2xy+ y 2 的值

Answer 1

3x

x+y -

x 2 - y 2

x-y •

3y

x 2 +2xy+ y 2

=

3x

x+y -

(x+y)(x-y)

x-y •

3y

(x+y) 2

=

3x

x+y -

3y

x+y

=

3x-3y

x+y ．

当

x

y =

1

3 时，y=3x．

∴原式=

3x-9x

x+3x = -

3

2 ．