[易错题汇编](一)集合逻辑篇

一、集合填空题:

(1)已知集合\(A = \left\{ {x \left|\right.- 2 \leqslant x \leqslant 7} \right\}\),\(B = \left\{ {x\left|\right.m + 1 < x < 2m-1} \right\}\) ,若\(A \cup B = A\),则实数\(m\)的取值范围是 ____;

(2)已知集合\(A = \left\{ {x \left|\right. {x^2} +px + 1 = 0 , p \in {\mathcal{R}}} \right\}\),若\(A \cap {{\mathcal{R}}_ + } = \varnothing \),则\(p\)的取值范围是 ____.

(3)设\(A = \left\{ {x \left|\right.{x^2} -3x +2= 0} \right\}\),\(B = \left\{ {x \left|\right.ax+1=0} \right\}\),若\(A \cap B = B\),则实数\(a\)组成的集合的真子集个数是 ____.


答案 (1)\(\left({-\infty,4} \right]\) ;(2)\( \left({-2,+ \infty } \right) \);(3)\(7\).

二、已知二次函数\(f\left(x \right)= a{x^2} + bx + c\)的两根$x_1,x_2$均大于\(3\) ,下列条件中,是该题设的充分必要条件的有_____.

① \(\begin{cases}
a\ne 0,\\{x_1} + {x_2} > 6 ,\\
{x_1} \cdot {x_2} > 9. \\
\end{cases}\)  

② \(\begin{cases}
a\ne 0,\\{x_1} + {x_2} > 6 ,\\
{x_1} \cdot {x_2} > 9, \\
\Delta \geqslant 0. \\
\end{cases}\)

③ \(\begin{cases} a\ne 0,\\\left({{x_1}-3} \right)+ \left({{x_2}-3} \right)> 0, \\ \left({{x_1}-3} \right)\cdot \left({{x_2}-3} \right)> 0, \\ \Delta \geqslant 0. \\ \end{cases}\)

④ \(\begin{cases} af\left(3 \right)> 0, \\ - \dfrac{b}{2a} > 3, \\ \Delta \geqslant 0 .\\ \end{cases}\) 

答案 ③④.

三、用“充分不必要”、“必要不充分”、“充要”、“既不充分也不必要”填空:

(1)\(\overrightarrow a\),\(\overrightarrow b \) 为非零向量,“  \(\overrightarrow a \perp \overrightarrow b \)”是“函数\(f\left( x \right) = \left( {x\overrightarrow a + \overrightarrow b } \right)\cdot \left( {x\overrightarrow b-\overrightarrow a } \right)\)  为一次函数”的 ____条件.

(2)\(\overrightarrow a\),\(\overrightarrow b \)为非零向量,“  \(\overrightarrow a\) , \(\overrightarrow b \)的夹角为钝角”是“\(\overrightarrow a \cdot \overrightarrow b < 0\)”的 ____条件.

(3)“\(x = \sqrt {ab} \) ”是“\(a\),\(x\) ,\(b\)”成等比数列的 ____条件.

(4)“\( - 4 < k < 0\)”是“\(\forall x \in {\mathcal{R}}\),\(k{x^2}-kx-1 < 0\)”的 ____ 条件.

(5)“数列\(\left\{ {a_n} \right\}\)是等比数列”是“数列\(\left\{ {{a_{2n-1}} + {a_{2n}}} \right\}\)是等比数列”的 ____条件.

答案 (1)必要不充分;

(2)充分不必要;

(3)既不充分也不必要;

(4)充分不必要;

(5)既不充分也不必要.

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