原式=[(a-3)/2(a-2)]÷{[5-(a+2)(a-2)]/(a-2)}
=[(a-3)/2(a-2)]÷[-(a²-9)]/(a-2)]
=[(a-3)/2(a-2)]×[-(a-2)/(a+3)(a-3)]
=-1/2(a+3)
=-1/(2a+6)