Maths Olympiad Prep

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Algebra Difficulty 5.6 AIME, harder Prove it Romania

Solve in real numbers the equation 2x1+21x=32^{x-1} + 2^{\frac{1}{\sqrt{x}}} = 3.

Solution

We notice that x>0x > 0. The given equation can be written as 2x+221x=62^x + 2 \cdot 2^{\frac{1}{\sqrt{x}}} = 6.

From the AM-GM inequality we have:
2x+21x+21x32x21x21x3=3213(x+2x),x>0. 2^x + 2^{\frac{1}{\sqrt{x}}} + 2^{\frac{1}{\sqrt{x}}} \ge 3 \cdot \sqrt[3]{2^x \cdot 2^{\frac{1}{\sqrt{x}}} \cdot 2^{\frac{1}{\sqrt{x}}}} = 3 \cdot 2^{\frac{1}{3}(x+\frac{2}{\sqrt{x}})}, \forall x > 0.
Also, using the AM-GM inequality, we have:
13(x+2x)x1x1x3=1, \frac{1}{3} \left( x + \frac{2}{\sqrt{x}} \right) \ge \sqrt[3]{x \cdot \frac{1}{\sqrt{x}} \cdot \frac{1}{\sqrt{x}}} = 1,
which implies that 2x+221x62^x + 2 \cdot 2^{\frac{1}{\sqrt{x}}} \ge 6. Therefore, we need equality in all AM-GM inequalities applied above, which is equivalent to x=1xx = \frac{1}{\sqrt{x}}, so x=1x = 1 is the unique solution of the given equation.

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