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NDA 2026
Exam
NDA
Year
2026
Subject
Mathematics
Chapter
Trigonometric Ratios & Identities

NDA 2026 · Mathematics · Trigonometric Ratios & Identities

If A and B are acute angles such that 2A+2B=π , then what is the...

If AA and BB are acute angles such that 2A+2B=π2A+2B=\pi, then what is the maximum value of sin⁡A⋅sin⁡B\sin A\cdot\sin B ?

Options

  1. A12\dfrac{1}{2}
  2. B14\dfrac{1}{4}
  3. C34\dfrac{\sqrt{3}}{4}
  4. D11

Correct answer

A. 12\dfrac{1}{2}

Step-by-step solution

Given 2A+2B=π2A + 2B = \pi, which simplifies to A+B=π2A + B = \dfrac{\pi}{2}.

Since AA and BB are acute angles, B=π2−AB = \dfrac{\pi}{2} - A.

The expression to maximize is sin⁡A⋅sin⁡B\sin A \cdot \sin B.

Substituting B=π2−AB = \dfrac{\pi}{2} - A, we get sin⁡A⋅sin⁡(π2−A)=sin⁡A⋅cos⁡A\sin A \cdot \sin\left(\dfrac{\pi}{2} - A\right) = \sin A \cdot \cos A.

Multiplying and dividing by 22, we obtain 2sin⁡Acos⁡A2=sin⁡2A2\dfrac{2\sin A \cos A}{2} = \dfrac{\sin 2A}{2}.

The maximum value of sin⁡2A\sin 2A is 11, which occurs when 2A=π22A = \dfrac{\pi}{2}, or A=π4A = \dfrac{\pi}{4}.

Therefore, the maximum value of sin⁡A⋅sin⁡B\sin A \cdot \sin B is 12×1=12\dfrac{1}{2} \times 1 = \dfrac{1}{2}.

Answer: 12\dfrac{1}{2}
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