即a²+3a+1=0
a²+3a=-1
原式=[(a+2)(a-2)/(a-2)²+1/(a-2)]/[2/a(a-2)]
=[(a+2)/(a-2)+1/(a-2)]×[a(a-2)/2]
=[(a+3)/(a-2)]×[a(a-2)/2]
=a(a+3)/2
=(a²+3a)/2
=-1/2