原式=[ (a?2) a(a+2) - a?1 (a+2)2 ]? a+2 a?4 = a?2 a(a?4) - a?1 (a+2)(a?4) = (a?2)(a+2)?(a?1)a a(a?4)(a+2) = a?4 a(a?4)(a+2) = 1 a(a+2) = 1 a2+2a ∵a2+2a-7=0，∴a2+2a=7，∴原式= 1 7 ．
