=(a^2+3a)/(a^2-9)/(a-3)=(a^2+3a)*(a-3)/(a^2-9)=(a^2+3a)/(a+3)=a(a+3)/(a+3)=a
1/3*(a^2+3a-18)=1/3*(a+6)(a-3)
没写清楚 麻烦请加一个括号
