先合并同类项 再求代数式的值:x²-2x²+5-x²-3,其中x=-1/2

x²-2x²+5-x²-3,
=-2x²+2
其中x=-1/2
原式子 = - 2 x ( 1/4) + 2
= - 1/2 + 2
= 2分之3

x²-2x²+5-x²-3
=（x²-2x²-x²）+（5-3）
=-2x²＋2
=-2×（-1/2）²＋2
=-1/2＋2
=3/2

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