Maths Olympiad Prep

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Algebra Difficulty 5.2 AIME, harder Prove it Philippines

Problem:
Find the range of the function f(x)=2x24x+1f(x) = 2^{x^{2} - 4x + 1}.

Solution

Solution:
Let y=x24x+1y = x^{2} - 4x + 1.

The expression x24x+1x^{2} - 4x + 1 is a quadratic in xx.

The minimum value of yy occurs at x=b2a=42=2x = -\frac{b}{2a} = \frac{4}{2} = 2.

At x=2x = 2:
y=(2)242+1=48+1=3 y = (2)^{2} - 4 \cdot 2 + 1 = 4 - 8 + 1 = -3

As x±x \to \pm \infty, y+y \to +\infty.

Therefore, y[3,)y \in [-3, \infty).

So, f(x)=2y=2x24x+1f(x) = 2^{y} = 2^{x^{2} - 4x + 1} takes all values 2t2^{t} where t[3,)t \in [-3, \infty).

Thus, the range is [23,)=[18,)[2^{-3}, \infty) = [\frac{1}{8}, \infty).

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.