COMEDK · Maths · 27. Application of Derivatives

If 3 cm/s is the rate at which the side of an equilateral triangle increases, then the rate of change of area, when the side is 12 cm is:

A \(18\sqrt{3} \ \text{cm}^2 / \text{s}\)
B \(18 \ \text{cm}^2 / \text{s}\)
C \(9\sqrt{3} \ \text{cm}^2 / \text{s}\)
D \(6\sqrt{3} \ \text{cm}^2 / \text{s}\)

Verified Solution

Answer & Solution

Correct Answer

(A) \(18\sqrt{3} \ \text{cm}^2 / \text{s}\)

Step-by-step Solution

Detailed explanation

Let the side of the equilateral triangle be \(a\) and its area be \(A\). The area of an equilateral triangle is given by \(A = \dfrac{\sqrt{3}}{4} a^2\). Differentiating with respect to time \(t\), we get:…

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

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