Application Of Derivatives Ques 85
85. A rectangular sheet of fixed perimeter with sides having their lengths in the ratio $8: 15$ is converted into an open rectangular box by folding after removing squares of equal area from all four corners. If the total area of removed squares is 100, the resulting box has maximum volume. The lengths of the sides of the rectangular sheet are
(2013 Adv.)
(a) 24
(b) 32
(c) 45
(d) 60
$e^{x} \quad, \quad 0 \leq x \leq 1$
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Answer:
Correct Answer: 85.(a, c)
Solution:
Formula:
Maxima and Minima of functions of one variable :
- PLAN
The problem is based on the concept to maximise volume of cuboid, i.e. to form a function of volume, say $f(x)$ find $f^{\prime}(x)$ and $f^{\prime \prime}(x)$. Put $f^{\prime}(x)=0$ and check $f^{\prime \prime}(x)$ to be + ve or -ve for minimum and maximum, respectively
Here, $\quad l=15 x-2 a, b=8 x-2 a$ and $h=a$
$\therefore \quad$ Volume $=(8 x-2 a)(15 x-2 a) a$
$ V=2 a \cdot(4 x-a)(15 x-2 a) ……(i) $
On differntiating Eq. (i) w.r.t $a$, we get
$ \frac{d v}{d a}=6 a^{2}-46 a x+60 x^{2} $
Again, differentiating,
$\frac{d^{2} v}{d a^{2}}=12 a-46 x$
Here, $ (\frac{d v}{d a})=0 \Rightarrow 6 x^{2}-23 x+15=0 $
At $\quad a=5 \quad \Rightarrow \quad x=3, \frac{5}{6}$
$\Rightarrow \quad (\frac{d^{2} v}{d a^{2}})=2(30-23 x)$
At $x=3, (\frac{d^{2} v}{d a^{2}})=2(30-69)<0$
$\therefore \quad$ Maximum when $x=3$, also at $x=\frac{5}{6} \Rightarrow (\frac{d^{2} v}{d a^{2}})>0$
$\therefore \quad$ At $x=5 / 6$, volume is minimum.
Thus, sides are $8 x=24$ and $15 x=45$
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