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Classical Inequalities
Your destination for questions of the form "do we know if this expression is always greater than this other expression?" Explore what humans know about mathematical inequalities.
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If the sum of two positive numbers is 18, what is the largest possible value of their product?
Hint. For non-negative numbers \(a\) and \(b,\) the Arithmetic Mean - Geometric Mean Inequality implies that \[\frac{a+b}{2} \geq \sqrt{ab},\] and \[\frac{a+b}{2} = \sqrt{ab} \text{ when } a = b.\]
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For values of \(x\) such that \( (8 +x)\) and \( (1 -x)\) are positive, what is the maximum possible value of
\[(8 +x)(1-x)?\]
Hint. Apply the Arithmetic Mean - Geometric Mean Inequality.
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If the sum of two legs of a right triangle is 20, what is the smallest possible value of the hypotenuse?
Hint. The quadratic mean, or root-mean-square of non-negative numbers \(a\) and \(b\) is greater than or equal to their arithmetic mean, i.e.
\[\sqrt{\frac{a^2+b^2}{2}} \geq \frac{a+b}{2},\]
with equality happening when \(a = b.\)
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What is the quadratic mean of the four values \(0, 0, 0, \) and \(8?\)
Note. The quadratic mean, or root-mean-square, of non-negative numbers \(x_1, \ldots, x_n\) is
\[\sqrt{\frac{x_1^2+\cdots + x_n^2}{n}}.\]
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