先化简,再求值1/2-a + 1/2+a - a²-4a/a²-4 ,其中a=-3

1/2-a + 1/2+a - a²-4a/a²-4 ,其中a=-3
1/(2-a) + 1/(2+a) - (a²-4a)/(a²-4)
=(2+a+2-a+a²-4a)/(4-a²)
=(4+a²-4a)/(4-a²)
=(2-a)²/(2-a)(2+a)
=(2-a)/(2+a)
=(2+3)/(2-3)
= -5
x-y/x²-2xy+y² - xy+y²/x²-y²,其中（x-2）²+ ly+3l=0
（x-2）²+ ly+3l=0
x-2=0且y+3=0
则,x=2 ,y = -3
x-y/x²-2xy+y² - xy+y²/x²-y²
=1/(x-y) - y/(x-y)
=(1-y)/(x-y)
=(1+3)/(2+3)
=4/5