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Question

If \(\theta\) lies in the fourth quadrant and \(3 \cot\theta + 4 = 0\), then what is the value of \(\sin 2\theta + \cos 2\theta\) ?

This question was previously asked in
NDA 2 2026 GAT Question Paper (13-Sep-2026)
The correct answer is
\(-\frac{17}{25}\)

Step 1: Simplify the given equation
3 cot θ + 4 = 0
3 cot θ = -4
cot θ = -4/3

Step 2: Find tan θ
Since tan θ = 1/cot θ,
tan θ = -3/4

Step 3: Determine values of sin θ and cos θ
Since θ is in the fourth quadrant, cos θ is positive and sin θ is negative.
Using the reference triangle with opposite = 3 and adjacent = 4:
Hypotenuse = √(3² + 4²) = √(9 + 16) = √25 = 5
Therefore:
sin θ = -3/5
cos θ = 4/5

Step 4: Calculate sin 2θ and cos 2θ using double-angle formulas
sin 2θ = 2 sin θ cos θ
sin 2θ = 2 × (-3/5) × (4/5) = -24/25

cos 2θ = cos² θ - sin² θ
cos 2θ = (4/5)² - (-3/5)² = 16/25 - 9/25 = 7/25

Step 5: Final Calculation
sin 2θ + cos 2θ = (-24/25) + (7/25)
sin 2θ + cos 2θ = -17/25

Final Answer: -17/25 (or -0.68)

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