(m^2-9)/[m(m-3)+m-3]
=(m+3)(m-3)/[(m+1)(m-3)]
=(m+3)/(m+1)分母的化解不太懂分子平方差公式哈：a²-9=a²-3²=(a+3)(a-3)分母提取公因式m-3，m(m-3)+m-3=m(m-3)+1*(m-3)=(m+1)(m-3)