5a²＋[a²＋（5a²-2a）-2(a2-3a)]，a=-二分之一 先化简再求值

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5a²＋闭罩[a²＋（5a²-2a）-2(a2-3a)]
=5a²+a²磨态吵+5a²-2a-2a²+6a
=9a²瞎侍+4a
当a=-1/2时
原式=9×(-1/2)²+4×(-1/2)
=9/4-2
=1/4

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