Question

A square piece of tin of side $24\, cm$ is to be made into a box without top by cutting a square from each corner and folding up the flaps to form a box. What should be the side of the square to be cut off so that the volume of the box is maximum?

• A. $2$
• B. $4$
• C. $6$
• D. $8$

Answer 1

Answer: (B)

$4$

Answer 2

Let $x$ cm be the length of a side of the square which is cut-off from each corner of the plate. Then, sides of the box as shown in figure are $24 - 2x, 24 - 2x$ and $x$. Let $V$ be the volume of the box. Then, $V = (24 - 2x) ^2 x = 4\,x ^3 - 96\,x ^2 + 576\,x$ $\Rightarrow \frac{dV}{dx} = 12x^{2} -192x + 576$ and $\frac{d^{2}V}{dx^{2}}$ $= 24x - 192$ For maximum or minimum values of $V$, we must have $\frac{dV}{dx} = 0$ $\Rightarrow 12 x^{2} - 192 x + 576 = 0$ $\Rightarrow x^{2} -16 x + 48 =0$ $\Rightarrow \left(x-12\right)\left(x-4\right) = 0$ $\Rightarrow x= 12, 4$ But, $x= 12$ is not possible Therefore, $x=4$. and $\left[ \frac{d^{2}V}{dx^{2}}\right]_{x=4} = -96 <0$ Hence, volume is maximum when $x = 4$.