Let x be a real number and let f ( x ) = 2 + ∣ x ∣ − x 2 .
For what values of x is f ( x ) positive?
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f ( x ) = 2 + ∣ x ∣ − x 2 = { 2 + x − x 2 2 − x − x 2 x ≥ 0 x < 0
We have two cases to analyze.
The first case: 2 + x − x 2 > 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
However, this function is only defined for x ≥ 0
Therefore, the range of values of x becomes 0 ≤ x < 2
The second case: 2 − x − x 2 > 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
However, this function is only defined for x < 0
Therefore, the range of values of x becomes − 2 < x < 0
Combine the two ranges of values of x , and you will get the final answer:
− 2 < x < 2
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First, we observe that all 3 components of f(x) -- 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