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Mathematics

If sin θ = (12)\Big(\dfrac{1}{2}\Big), then the value of (15cot2θ+15)\Big(\dfrac{1}{5} \cot^2\theta + \dfrac{1}{5}\Big) is:

  1. (15)\Big(\dfrac{1}{5}\Big)

  2. (45)\Big(\dfrac{4}{5}\Big)

  3. (1125)\Big(\dfrac{1}{125}\Big)

  4. 25

Trigonometric Identities

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Answer

Solving,

(15cot2θ+15)15(cot2θ+1)15(cosec2θ).\Rightarrow \Big(\dfrac{1}{5} \cot^2\theta + \dfrac{1}{5}\Big) \\[1em] \Rightarrow \dfrac{1}{5} \Big(\cot^2\theta + 1 \Big) \\[1em] \Rightarrow \dfrac{1}{5} (\cosec^2 \theta).

We know that,

cosecθ=1sinθcosecθ=112=2cosec2θ=22=415cosec2θ15(4)45.\cosec \theta = \dfrac{1}{\sin \theta } \\[1em] \Rightarrow \cosec \theta = \dfrac{1}{\dfrac{1}{2}} = 2\\[1em] \Rightarrow \cosec^2 \theta = 2^2 = 4 \\[1em] \Rightarrow \dfrac{1}{5}\cosec^2 \theta \\[1em] \Rightarrow \dfrac{1}{5}(4) \\[1em] \Rightarrow \dfrac{4}{5}.

Hence, option 2 is the correct option.

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