Question : If $(\sin \theta+\operatorname{cosec} \theta)^2+(\cos \theta+\sec \theta)^2=k+\tan ^2 \theta+\cot ^2 \theta$, then the value of $k$ is equal to:
Option 1: 7
Option 2: 2
Option 3: 5
Option 4: 9
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Correct Answer: 7
Solution :
Given: $(\sin \theta+\operatorname{cosec} \theta)^2+(\cos \theta+\sec \theta)^2=k+\tan ^2 \theta+\cot ^2 \theta$
⇒ $\sin^{2}θ + 2\sinθ\operatorname{cosec}θ + \operatorname{cosec}^{2}θ +\cos^{2}θ + 2\cos θ \secθ + \sec^{2}θ=k+\tan ^2 \theta+\cot ^2 \theta$
⇒ $k = \sin^{2}θ + \operatorname{cosec}^{2}θ + \cos^{2}θ+\sec^{2}θ + 2(1+1) - \tan ^2 \theta-\cot ^2 \theta$
⇒ $k = \sin^{2}θ + \cos^{2}θ + \operatorname{cosec}^{2}θ +\sec^{2}θ + 4 - \tan ^2 \theta-\cot ^2 \theta$
⇒ $k = 1 + \operatorname{cosec}^{2}θ -\cot ^2 \theta +\sec^{2}θ - \tan ^2 \theta + 4$
⇒ $k = 1 + 1 + 1 + 4$
⇒ $k = 7$
Hence, the correct answer is 7.
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