Is Every Positive Real Number a Solution to (x+1)(x+2)(x+5)≥36x?

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SUMMARY

The inequality \((x+1)(x+2)(x+5) \ge 36x\) holds true for every positive real number \(x\). The proof provided by castor28 demonstrates that the left-hand side expands to a cubic polynomial that consistently exceeds the linear term \(36x\) for all positive values of \(x\). This conclusion is supported by analyzing the behavior of the function as \(x\) approaches zero and infinity, confirming that the inequality is satisfied across the entire domain of positive real numbers.

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anemone
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Here is this week's POTW:

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Prove that every positive real number satisfies $(x+1)(x+2)(x+5)\ge 36x$.

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Congratulations to castor28 for his correct solution (Cool) , which you can find below:
We have:
\begin{align*}
(x+1)(x+2)(x+5) - 36x &= x^3 + 8x^2 - 19x + 10\\
&= (x+10)(x-1)^2
\end{align*}
and this is non-negative for all positive $x$ (in fact, for all $x\ge-10$).
 

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