If 4sinθ + cosecθ = 4, 0° < θ < 90°, then the value of sin3θ + cos3θ is
- AOption (a)
1
- BOption (b)
0
- COption (c)
-1
- DOption (d)
2
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Correct answer
1
Why this answer is correct
Shortcut Trick
The equation 4sinθ + 1/sinθ = 4 is in the form x + 1/x = 2 where x = 2sinθ.
This standard form implies x = 1, which means 2sinθ = 1 or sinθ = 1/2.
For the range 0° < θ < 90°, the value of θ is 30°.
Substitute θ = 30° in the required expression: sin(3 × 30°) + cos(3 × 30°).
sin 90° + cos 90° = 1 + 0 = 1.
∴ The correct answer is 1.
Alternate Method
Given: 4sinθ + cosecθ = 4, where 0° < θ < 90°
Formula Used: cosecθ = 1/sinθ and (a − b)2 = a2 − 2ab + b2
Calculations:
⇒ 4sinθ + 1/sinθ = 4
⇒ (4sin2θ + 1) / sinθ = 4
⇒ 4sin2θ + 1 = 4sinθ
⇒ 4sin2θ − 4sinθ + 1 = 0
⇒ (2sinθ − 1)2 = 0
⇒ 2sinθ − 1 = 0
⇒ sinθ = 1/2
⇒ Since sin 30° = 1/2 and 0° < θ < 90°, then θ = 30°.
⇒ Required value = sin 3θ + cos 3θ
⇒ sin(3 × 30°) + cos(3 × 30°)
⇒ sin 90° + cos 90°
⇒ 1 + 0 = 1
∴ The correct answer is 1.
Additional Information
Reciprocal Identities
The trigonometric functions sinθ and cosecθ are reciprocals, meaning sinθ × cosecθ = 1.
Standard Trigonometric Values
Key values include sin 30° = 1/2, sin 90° = 1, and cos 90° = 0.
Complementary Angles
For any angle A, sin A = cos(90° − A) and cos A = sin(90° − A).
