Class CBSE Class 12 Mathematics Applications of Derivatives Q #1262
COMPETENCY BASED
UNDERSTAND
5 Marks 2024 AISSCE(Board Exam) LA
The perimeter of a rectangular metallic sheet is 300 cm. It is rolled along one of its sides to form a cylinder. Find the dimensions of the rectangular sheet so that volume of cylinder so formed is maximum.

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Detailed Solution

Step 1: Define Variables and Constraints

Let the length and breadth of the rectangular sheet be $l$ and $b$ respectively. The perimeter is given as 300 cm, so we have: $2(l + b) = 300$ $l + b = 150$ $b = 150 - l$

Step 2: Express Volume in Terms of One Variable

When the sheet is rolled along its length $l$, the circumference of the base of the cylinder is $l$, and the height of the cylinder is $b$. Thus, $2\pi r = l$ $r = \frac{l}{2\pi}$ The volume $V$ of the cylinder is given by: $V = \pi r^2 h = \pi (\frac{l}{2\pi})^2 b = \pi \frac{l^2}{4\pi^2} (150 - l) = \frac{l^2(150 - l)}{4\pi}$

Step 3: Maximize the Volume

To maximize the volume, we need to find the critical points by taking the derivative of $V$ with respect to $l$ and setting it to zero. $\frac{dV}{dl} = \frac{1}{4\pi} \frac{d}{dl} (150l^2 - l^3) = \frac{1}{4\pi} (300l - 3l^2)$ Set $\frac{dV}{dl} = 0$: $300l - 3l^2 = 0$ $3l(100 - l) = 0$ So, $l = 0$ or $l = 100$. Since $l = 0$ would result in zero volume, we consider $l = 100$.

Step 4: Verify Maximum Volume

To verify that $l = 100$ gives a maximum volume, we take the second derivative of $V$ with respect to $l$: $\frac{d^2V}{dl^2} = \frac{1}{4\pi} (300 - 6l)$ Evaluate at $l = 100$: $\frac{d^2V}{dl^2} = \frac{1}{4\pi} (300 - 600) = \frac{-300}{4\pi} = \frac{-75}{\pi}$ Since $\frac{d^2V}{dl^2} < 0$, the volume is maximized when $l = 100$.

Step 5: Find the Dimensions

When $l = 100$, we have $b = 150 - l = 150 - 100 = 50$. Therefore, the dimensions of the rectangular sheet are length $l = 100$ cm and breadth $b = 50$ cm.

Final Answer: Length = 100 cm, Breadth = 50 cm

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Pedagogical Audit
Bloom's Analysis: This is an APPLY question because it requires students to apply the concepts of optimization (calculus) to a real-world problem of maximizing the volume of a cylinder formed from a rectangular sheet.
Knowledge Dimension: PROCEDURAL
Justification: The question requires a series of steps to solve, including setting up the problem, finding the derivative, setting the derivative to zero, and verifying the maximum. This involves knowing the procedures for optimization.
Syllabus Audit: In the context of CBSE Class 12, this is classified as COMPETENCY. It assesses the student's ability to apply calculus concepts to solve a practical problem, aligning with competency-based education principles.