Class CBSE Class 12 Mathematics Applications of Derivatives Q #1700
COMPETENCY BASED
APPLY
1 Marks 2026 AISSCE(Board Exam) MCQ SINGLE
The surface area of a sphere when its volume changes at the same rate as its radius is :
(A) $4\pi$ sq. units
(B) 1 sq. unit
(C) 4 sq. units
(D) $\pi$ sq. units
Correct Answer: B

AI Tutor Explanation

Powered by Gemini

Detailed Solution

Step 1: Define the given variables and rates

Let $r$ be the radius of the sphere. The volume $V$ of the sphere is given by $V = \frac{4}{3}\pi r^3$. The surface area $S$ of the sphere is given by $S = 4\pi r^2$. We are given that the rate of change of volume with respect to time is equal to the rate of change of radius with respect to time, i.e., $\frac{dV}{dt} = \frac{dr}{dt}$.

Step 2: Differentiate volume with respect to time

Using the chain rule, differentiate $V$ with respect to $t$: $$ \frac{dV}{dt} = \frac{d}{dt} \left( \frac{4}{3}\pi r^3 \right) = 4\pi r^2 \frac{dr}{dt} $$

Step 3: Solve for the radius

Substitute the given condition $\frac{dV}{dt} = \frac{dr}{dt}$ into the equation: $$ \frac{dr}{dt} = 4\pi r^2 \frac{dr}{dt} $$ Assuming $\frac{dr}{dt} \neq 0$, we divide both sides by $\frac{dr}{dt}$: $$ 1 = 4\pi r^2 $$

Step 4: Calculate the surface area

The surface area $S$ is $4\pi r^2$. From the equation derived in Step 3, we already have $4\pi r^2 = 1$. Therefore, the surface area is 1 sq. unit.

Final Answer: 1 sq. unit

AI generated content. Review strictly for academic accuracy.

Pedagogical Audit
Bloom's Analysis: This is an APPLY question because the student must translate a verbal condition into a mathematical differential equation and apply the chain rule to solve for the unknown variable.
Knowledge Dimension: PROCEDURAL
Justification: The question requires the execution of a specific sequence of steps involving differentiation and algebraic manipulation to reach the solution.
Syllabus Audit: In the context of CBSE Class 12, this is classified as COMPETENCY. This question tests the application of 'Application of Derivatives' (Rate of Change) which is a core competency area in the Class 12 Mathematics curriculum.
||KEY:B