2a+6分之a+15-a^2+6a+9分之a^2-81除以a+3分之a-9

原式=[(a+15)/(2a+6)]-[(a²-81)/(a²+6a+9)]÷[(a-9)/(a+3)]
=[(a+15)/(2a+6)]-[(a+9)(a-9)/(a+3)²]×[(a+3)/(a-9)]
=[(a+15)/(2a+6)]-[(a+9)/(a+3)]
={ (a+15)/[2(a+3)] }-{ 2(a+9)/[2(a+3)] }
=[(a+15)-2(a+9)]/[2(a+3)]
=(a+15-2a-18)/[2(a+3)]
=(-a-3)/[2(a+3)]
=-(a+3)/[2(a+3)]
=-1/2