Maths Olympiad Prep

Track / Stage 3 / 181 of 260 #181 of 1964

Problem 181

AMC 10/12, early questions
Algebra Difficulty 3.5 Find the answer

Let M={xx2}M = \left\{ x \mid x \geq -2 \right\} and N={x2x1>0}N = \left\{ x \mid 2^x - 1 > 0 \right\}. Find the intersection of set MM and the complement of set NN in the real numbers, MCRNM\cap C_{\mathbb{R}}N.

A number or a short expression. Spacing, $ signs and \frac vs / are all fine.

Official solution

To solve the problem, we need to understand the definition of sets MM and NN and find their intersection after determining the complement of set NN with respect to the real numbers.

Firstly, the set MM is defined as all xx that are greater than or equal to 2-2. In interval notation, we have:
M=[2,+)M = [ -2, +\infty )

For set NN, we need to find all xx that satisfy 2x1>02^x - 1 > 0. Solving for xx yields:
2x>12^x > 1
This inequality is true for all x>0x > 0, as any positive exponent of 2 results in a number greater than 1.

Therefore, we have:
N=(0,+)N = ( 0, +\infty )

Now, the complement of NN in the real numbers, CRNC_{\mathbb{R}}N), consists of all real numbers that are not in NN. Hence, it includes all real numbers less than or equal to 00. Given this, we can express CRNC_{\mathbb{R}}N as:
CRN=(,0]C_{\mathbb{R}}N = ( -\infty, 0 ]

Now we need to find MCRNM \cap C_{\mathbb{R}}N, the intersection of sets MM and CRNC_{\mathbb{R}}N. This will be all numbers that are in both sets. As MM includes numbers from 2-2 upward, and CRNC_{\mathbb{R}}N includes numbers up to 00, their intersection will include all numbers from 2-2 up to 00, inclusive.

Therefore:
MCRN=[2,0]M \cap C_{\mathbb{R}}N = [-2, 0]

Hence, the correct answer is:

\boxed{D: \left\{ x \mid -2 \leq x \leq 0 \right\}}

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