计算:3/(a2-9)-7/(a2+4a-21)

计算:3/(a2-9)-7/(a2+4a-21)
计算:3/(a2-9)-7/(a2+4a-21)

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)]
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