2分之1a的2次方b-5ac-（3a的2次方-a的2次方b）+（3ac-4a的2次方c）
=(a²b-5ac)/2-2(3a²-a²b)/2+2(3ac-4a²c)/2
=(a²b-5ac-6a²+2a²b+6ac-8a²c)/2
=(3a²b+ac-6a²-8a²c)/2
=[3a²(b-2)+ac(1-8a)]/2
={3a²(2-2)+(-1)(-2)[1-8*(-1)]}/2
=[0+2*(1+8)]/2
=2*9/2
=9
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