Question
Download Solution PDFWhat is the value of sin 75° + sin 15°?
Answer (Detailed Solution Below)
Detailed Solution
Download Solution PDFGiven:
sin 75° + sin 15°
Concept used:
1. 2 × sin θ × cos θ = sin 2θ
2. sin (90° - θ) = cos θ
3. sin 30° = 1/2
4. sin2 θ + cos2 θ = 1
Calculation:
sin 75° + sin 15°
⇒ sin (90° - 15°) + sin 15°
⇒ cos 15° + sin 15°
⇒ sin 15° + cos 15° = k (let)
According to the concept,
sin2 15° + cos2 15° = 1
⇒ (sin 15° + cos 15°)2 - 2.sin 15°.cos 15° = 1
⇒ k2 - sin (2 × 15)° = 1
⇒ k2 - sin 30° = 1
⇒ k2 - 1/2 = 1
⇒ k2 = 1 + 1/2
⇒ k2 = 3/2
⇒ k = \(\sqrt{\frac{3}{2}}\)
∴ The value of sin 75° + sin 15° is \(\sqrt{\frac{3}{2}}\).
Shortcut Trick
We know, sin C + sin D = \(2 \frac{sin~(C+D)}{2}\frac{cos~(C-D)}{2}\)
sin 75° + sin 15 = \(2 sin~\frac{(75+15)}{2}cos\frac{(75-15)}{2}\) = 2 × sin 45° × cos 30° = 2 × \(\frac{1}{\sqrt{2}}\) × \(\frac{\sqrt{3}}{2}\) = \(\sqrt{\frac{3}{2}}\)
Last updated on Jul 19, 2025
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