(a二次方-4/a二次方-4a+4-1/2-a）÷2/a二次方-2a
=[（a-2）（a+2）/(a-2)²-1/(2-a)]×a（a-2）/2
=[(a+2)/(a-2)+1/(a-2)]×a（a-2）/2
=a(a+3)/2=（a²+3a）/2
因为a满足方程a二次方+3a+1=0
所以
a²+3a=-1
即
原式=-1/2