Think positive

Algebra Level 2

Let x x be a real number and let f ( x ) = 2 + x x 2 f(x) = 2 + |x| - x^2 .

For what values of x x is f ( x ) f(x) positive?

2 < x -2 < x It is positive for all values of x x x < 2 x < 2 2 < x < 2 -2 < x < 2

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2 solutions

Denton Young
Mar 24, 2016

First, we observe that all 3 components of f(x) -- 2 , x , x 2 2, |x|, x^2 -- are symmetric around x =0. So f(-x) = f(x).

Therefore, we only need to look at the case where x > 0 (at 0 itself, f(x) is positive as the two components involving x vanish, leaving f(0) = 2.) For x > 0, |x| = x.

We observe that f'(x) = 1 - 2x and f''(x) = -2. so the function starts positive and once it becomes negative, it never becomes positive again.

For positive x, factorizing yields (2-x)(x-1) = 0 if we're trying to find zeroes. So x = -1 (impossible) or x = 2. The function turns negative as it crosses x = 2, and hence when it crosses x = -2 as well.

So the range where it is positive is -2 < x < 2

Hung Woei Neoh
May 10, 2016

f ( x ) = 2 + x x 2 = { 2 + x x 2 x 0 2 x x 2 x < 0 f(x) = 2 + |x| - x^2 =\begin{cases} 2 + x - x^2 & \quad\quad x \geq 0\\ 2 - x - x^2 & \quad\quad x<0\\ \end{cases}

We have two cases to analyze.

The first case: 2 + x x 2 > 0 2+x-x^2 > 0

( 2 x ) ( 1 + x ) > 0 (2-x)(1+x) > 0

Draw out a number line or the graph of this quadratic function, and you should get the range of:

1 < x < 2 -1<x<2

However, this function is only defined for x 0 x \geq 0

Therefore, the range of values of x x becomes 0 x < 2 0 \leq x < 2

The second case: 2 x x 2 > 0 2-x-x^2 > 0

( 2 + x ) ( 1 x ) > 0 (2+x)(1-x) > 0

Draw out a number line or the graph of this quadratic function, and you should get the range of:

2 < x < 1 -2<x<1

However, this function is only defined for x < 0 x < 0

Therefore, the range of values of x x becomes 2 < x < 0 -2 < x < 0

Combine the two ranges of values of x x , and you will get the final answer:

2 < x < 2 \boxed{-2 < x < 2}

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