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【简答题】
已知以下四个命题： ①如果x 1 ，x 2 是一元二次方程ax 2 +bx+c=0的两个，且x 1 ＜x 2 ，那么不等式ax 2 +bx+c＜0的{x|x 1 ＜x＜x 2 }．②若 x-1 x-2 ≤0 ，则（x-1）（x-2）≤0．③“若M={-1，0，1}，则x 2 -2x+m＞0的是实数集R”的逆否命题．④若函数f（x）在（-∞，+∞）上递增，且a+b≥0，则f（a）+f（b）≥f（-a）+f（-b）． 其中为真命题的是______（填上你认为正确的序号）．
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