Class CBSE Class 12 Mathematics Applications of Derivatives Q #1701
COMPETENCY BASED
APPLY
1 Marks 2026 AISSCE(Board Exam) MCQ SINGLE
The rate of change of volume of a sphere with respect to its diameter, when its radius is 5 cm, is:
(A) $400\pi~cm^{3}/cm$
(B) $100\pi~cm^{3}/cm$
(C) $50\pi~cm^{3}/cm$
(D) $25\pi~cm^{3}/cm$
Correct Answer: C

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Detailed Solution

Step 1: Define the variables

Let $r$ be the radius and $x$ be the diameter of the sphere. We know that $x = 2r$, which implies $r = \frac{x}{2}$.

Step 2: Express Volume in terms of diameter

The volume $V$ of a sphere is given by $V = \frac{4}{3}\pi r^3$. Substituting $r = \frac{x}{2}$ into the formula: $$V = \frac{4}{3}\pi \left(\frac{x}{2}\right)^3 = \frac{4}{3}\pi \left(\frac{x^3}{8}\right) = \frac{\pi x^3}{6}$$

Step 3: Differentiate with respect to diameter

We need to find the rate of change of volume with respect to diameter, which is $\frac{dV}{dx}$: $$\frac{dV}{dx} = \frac{d}{dx} \left(\frac{\pi x^3}{6}\right) = \frac{\pi}{6} \cdot 3x^2 = \frac{\pi x^2}{2}$$

Step 4: Evaluate at the given radius

Given $r = 5$ cm, the diameter $x = 2r = 10$ cm. Substituting $x = 10$ into the derivative: $$\frac{dV}{dx} = \frac{\pi (10)^2}{2} = \frac{100\pi}{2} = 50\pi$$

Final Answer: 50\pi~cm^{3}/cm

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Pedagogical Audit
Bloom's Analysis: This is an APPLY question because it requires the student to translate a geometric relationship into a calculus-based rate of change problem and perform substitution.
Knowledge Dimension: PROCEDURAL
Justification: The student must follow a specific sequence of steps: variable substitution, differentiation, and evaluation.
Syllabus Audit: In the context of CBSE Class 12, this is classified as COMPETENCY. This question tests the application of the 'Application of Derivatives' chapter, specifically the concept of related rates, which is a core competency in the NCERT curriculum.