Question

先化简

-2xy

x2-y2

÷(

1

x-y

-

1

x+y

)，再求值，其中x=1-

2

．

Answer 1

原式=

-2xy

(x+y)(x-y)

÷[

x+y

(x+y)(x-y)

-

x-y

(x+y)(x-y)

]
=

-2xy

(x+y)(x-y)

÷

2y

(x+y)(x-y)

=

-2xy

(x+y)(x-y)

•

(x+y)(x-y)

2y

=-x，
当x=1-

2

时，原式=-（1-

2

）=

2

-1．