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Algebra Difficulty 3.0 AMC 10/12 Find the answer

Given the set A={x2x5}A=\{x|-2\leqslant x\leqslant 5\} and the set B={xp+1x2p1}B=\{x|p+1\leqslant x\leqslant 2p-1\}, find the range of values for the real number pp such that AB=BA\cap B=B.

A number or a short expression. Spacing and $ signs are ignored.

Solution

Solution: According to the problem, if AB=BA\cap B=B, then BAB\subseteq A. We will discuss this in three cases:

1. When p+1>2p1p+1 > 2p-1, or equivalently p2p 2, we have B={xp+1x2p1}B=\{x|p+1\leqslant x\leqslant 2p-1\}. If BAB\subseteq A, then we have the system of inequalities {2p15p+12\begin{cases} 2p-1\leqslant 5 \\\\ p+1\geqslant -2\end{cases}. Solving this system, we get (3p3)(-3\leqslant p\leqslant 3). However, since p>2p > 2, the condition is satisfied when 2<p32 < p\leqslant 3.

In summary, the condition AB=BA\cap B=B holds when p3p\leqslant 3. Therefore, the range of values for pp is p3\boxed{p\leqslant 3}.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.