MCQ
A box open from top is made from a rectangular sheet of dimension $\mathrm{a} \times \mathrm{b}$ by cutting squares each of side $x$ from each of the four corners and folding up the flaps. If the volume of the box is maximum, then $\mathrm{x}$ is equal to :
  • A
    $\frac{a+b-\sqrt{a^{2}+b^{2}-a b}}{12}$
  • B
    $\frac{a+b-\sqrt{a^{2}+b^{2}+a b}}{6}$
  • $\frac{a+b-\sqrt{a^{2}+b^{2}-a b}}{6}$
  • D
    $\frac{a+b+\sqrt{a^{2}+b^{2}-a b}}{6}$

Answer

Correct option: C.
$\frac{a+b-\sqrt{a^{2}+b^{2}-a b}}{6}$
c
$\mathrm{V}=\ell . \mathrm{b} \cdot \mathrm{h}=(\mathrm{a}-2 \mathrm{x})(\mathrm{b}-2 \mathrm{x}) \mathrm{x}$

$\Rightarrow \mathrm{V}(\mathrm{x})=(2 \mathrm{x}-\mathrm{a})(2 \mathrm{x}-\mathrm{b}) \mathrm{x}$

$\Rightarrow \mathrm{V}(\mathrm{x})=4 \mathrm{x}^{3}-2(\mathrm{a}+\mathrm{b}) \mathrm{x}^{2}+\mathrm{abx}$

$\Rightarrow \frac{\mathrm{d}}{\mathrm{dx}} \mathrm{v}(\mathrm{x})=12 \mathrm{x}^{2}-4(\mathrm{a}+\mathrm{b}) \mathrm{x}+\mathrm{ab}$

$\frac{\mathrm{d}}{\mathrm{dx}}(\mathrm{v}(\mathrm{x}))=0 \Rightarrow 12 \mathrm{x}^{2}-4(\mathrm{a}+\mathrm{b}) \mathrm{x}+\mathrm{ab}=0<_{\beta}^{\alpha}$

$\Rightarrow \mathrm{x}=\frac{4(\mathrm{a}+\mathrm{b}) \pm \sqrt{16(\mathrm{a}+\mathrm{b})^{2}-48 \mathrm{ab}}}{2(12)}$

$=\frac{(a+b) \pm \sqrt{a^{2}+b^{2}-a b}}{6}$

Let $\mathrm{x}=\alpha=\frac{(\mathrm{a}+\mathrm{b})+\sqrt{\mathrm{a}^{2}+\mathrm{b}^{2}-\mathrm{ab}}}{\frac{6}{\mathrm{~d}}}$

$\beta=\frac{(\mathrm{a}+\mathrm{b})-\sqrt{\mathrm{a}^{2}+\mathrm{b}^{2}-\mathrm{ab}}}{6}$

Now, $12(\mathrm{x}-\alpha)(\mathrm{x}-\beta)=0$

$\therefore \mathrm{x}=\beta$

$=\frac{\mathrm{a}+\mathrm{b}-\sqrt{\mathrm{a}^{2}+\mathrm{b}^{2}-\mathrm{ab}}}{\mathrm{b}}$

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