# Homework Help: Finding the Complement of a Set

1. May 28, 2015

### Keen94

1. Find P'={x I ~px} for the given open sentences px.
#25. x2≥4.
(Problem from 1.10, Principles of Mathematics by Allendoerfer and Oakley.
Solution offered at the back of the book: {x I -4<x<4}.

2. Relevant equations
If P={x∈ℝ I px} then P'={x∈ℝ I ~px}

3. The attempt at a solution
x2≥4 ⇒ x≤-2 or x≥2.
P={x∈ℝ I x≤-2 or x≥2}. P'={x∈ℝ I -2<x<2}

The original proposition is true when a number is equal to or less than -2. It is equally true when it is equal to or greater than 2. If we negate the proposition then the elements of this set will be the ones not found in the original set. This leaves the interval (-2,2). I don't understand why the interval would be (-4,4) as the solution found at the back of the book suggests. BTW First Post!

2. May 28, 2015

### SammyS

Staff Emeritus
Hello Keen94. Welcome to PF !

The book must have a typo. Your analysis is correct !

3. May 28, 2015

### Keen94

Thank you for the speedy reply!