计算:3/(a2-9)-7/(a2+4a-21)

3/(a²-9)-7/(a²+4a-21)=3/[(a+3)(a-3)]-7/[(a+7)(a-3)]=3(a+7)/[(a+3)(a-3)(a+7)]-7(a+3)[(a+3)(a-3)(a+7)]=[3(a+7)-7(a+3)]/[(a+3)(a-3)(a+7)]=(3a+21-7a-21)/[(a+3)(a-3)(a+7)]=4a/[(a+3)(a-3)(a+7)]如...���Ǽ��㣬����ķ�ĸ��Ҫ�˿�����������ʦ��ôҪ��ģ��Ͼ��Ҳ�����ѧ��ʦ�����Ǻ��˽���ѧ���