<s>[INST] If $x$ is a real and $0 < x < 4$, then $\frac{4}{x(4-x)} \geq 1$ I got this exercise from the textbook Book of Proof, CH4 E12. I've tackle this problem in the following manner:
Suppose x is a real and $0 < x < 4$, it follows that,
\begin{align*}
&\Rightarrow 0 - 2 < x - 2 < 4 - 2 \\
&\Rightarrow     4 < (x - 2)^2 < 4\\
&\Rightarrow     0 \leq (x - 2)^2 < 4
\end{align*}
Since, $x(4 - x) = 4x - x^2 = 4 - (x - 2)^2$, then
$$\dfrac{4}{x(4 - x)} = \dfrac{4}{4 - (x - 2)^2}.$$
This expression is greater or equal to $1$ for
$0 \leq (x - 2)^2 < 4$. Thus,
$$\dfrac{4}{x(4 - x)} \geq 1.$$
I'm quite new to proof technique and I'm using this book to self-learn logic and proofing writing. My question is: is the solution stated above logically sound? Would my arguments be considered sufficient to prove that $P \Rightarrow Q$?
 [/INST] Hint See that: $0\le x\le 4 $ implies $0\le 4-x\le 4$ and then,
$$0\le x(4-x) =-(x-2)^2+4\le4$$
Thus directly implies your inequality.$$\dfrac{4}{x(4 - x)} \geq 1.$$
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