Question

下列命题中真命题的是（ ） ①若a+b＞0且ab＞0，则a＞0且b＜0. ②若a＞b且ab＞0，则a＞b＞0 ③在同

下列命题中真命题的是（ ）。
①若a+b＞0且ab＞0，则a＞0且b＜0.
②若a＞b且ab＞0，则a＞b＞0
③在同一平面内，垂直于同一直线的两直线平行。
④一个锐角的补角比它的余角小90°。

Answer 1

假，假，真，假

①若a+b＞0且ab＞0，则a＞0且b＜0.
假，原因：ab>0，说明a、b同号，所以已知成立，则结论不成立
②若a＞b且ab＞0，则a＞b＞0
假，原因：ab>0，说明a、b同号；若a、b同为正，则a>b>0，若a、b同为负，则应为b<a<0
③在同一平面内，垂直于同一直线的两直线平行。
真（前提：同一平面内的两直线，通常是指不重合的两直线；该结论，对于空间则不一定成立）
④一个锐角的补角比它的余角小90°。
假，令0<A<90°，则其补角为90°<B=180°-A<180°，其余角为0<C=90°-A<90°
则其补角与余角的差为：B-C=（180°-A）-（90°-A）=90°
因此，应该是“一个锐角的补角比它的余角小90°。”

Answer 2

3

Answer 3

1错ab大于0，a，b要同号 2错.a大于b 可以两个都为负数 3. 对 4. 错 小字改为大

Answer 4

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