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  x-y/x²-2xy+y²  -  xy+y²/x²-y²,其中（x-2）²+  ly+3l=0

1/(2-a)+1/(2+a)-(a^2-4a)/(a^2-4)
=4/(4-a^2)+(a^2+4a)/(4-a^2)
=[(a+2)^2]/(4-a^2)
=(a+2/(2-a) 把a=-3代入得 -1/5
(x-y)/(x^2-2xy+y^2)-(xy+y^2)/(x^2-y^2)
=1/(x-y)-y/(x-y)
=(1-y)/(x-y) 因为（x-2）²+ |y+3|=0 所以 x=2 y=-3 并且把值代入式子中得4/5

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