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Question

If A and B are acute angles such that \(2A + 2B = \pi\), then what is the maximum value of \(\sin A \cdot \sin B\) ?

This question was previously asked in
NDA 2 2026 GAT Question Paper (13-Sep-2026)
The correct answer is

1/2

Conditions and Objective

Given:

  • A and B are acute angles, meaning \(0 < A < \frac{\pi}{2}\) and \(0 < B < \frac{\pi}{2}\).
  • The constraint is \(2A + 2B = \pi\).

Simplifying the constraint gives \(2(A+B) = \pi\), which implies \(A+B = \frac{\pi}{2}\). Angles A and B are complementary.

The objective is to find the maximum value of the product \(P = \sin A \cdot \sin B\).

Derivation Using Identities

Since \(A+B = \frac{\pi}{2}\), we have \(B = \frac{\pi}{2} - A\). Substitute this into the expression for P:

  • \(\sin B = \sin\left(\frac{\pi}{2} - A\right) = \cos A\).
  • Therefore, \(P = \sin A \cdot \cos A\).

Using the trigonometric double angle identity \(\sin(2A) = 2 \sin A \cos A\), we can write:

  • \(P = \frac{1}{2} (2 \sin A \cos A) = \frac{1}{2} \sin(2A)\).

Maximum Value Calculation

We need to find the maximum value of \(P = \frac{1}{2} \sin(2A)\) considering the range of A:

  • Since A is an acute angle (\(0 < A < \frac{\pi}{2}\)), the angle \(2A\) lies in the interval \((0, \pi)\).
  • The sine function, \(\sin(x)\), achieves its maximum value of 1 within the interval \((0, \pi)\).
  • The maximum occurs when \(2A = \frac{\pi}{2}\), which gives \(A = \frac{\pi}{4}\).
  • If \(A = \frac{\pi}{4}\), then \(B = \frac{\pi}{2} - \frac{\pi}{4} = \frac{\pi}{4}\). Both angles are indeed acute.

The maximum value based on this derivation is:

  • \(P_{max} = \frac{1}{2} \times (\text{maximum value of } \sin(2A)) = \frac{1}{2} \times 1 = \frac{1}{2}\).

Final Answer Selection

The mathematically derived maximum value is 1/2. However, selecting from the given options, the designated correct answer is Option B.

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