InterviewSolution
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If cosec θ + cot θ = 2, then cos θ =A) 3/5B) 4/5C) 5/3D) 6/5 |
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Answer» Correct option is: A) \( \frac{3}{5}\) We have cosec \(\theta\) + cot \(\theta\) = 2 = \(\frac {1}{sin \theta} + \frac {cos \theta}{sin \theta} =2\) = 1 + cos \(\theta\) = 2 sin \(\theta\) = \((1 + cos \theta )^2 = 4\, sin^2\theta\) (By squaring both sides) = 1 + \(cos^2\theta + 2\, cos \theta = 4 (1- cos^2\theta)\) (\(\because\) \(sin^2\theta = 1- cos^2\theta\)) = \(5 \,cos ^2\theta + 2\, cos \theta - 3 = 0\) = \(5 \,cos ^2\theta + 5 \,cos\theta - 3 \, cos\theta - 3 = 0\) = \(5 \,cos\theta (cos\theta +1) - 3 (cos\theta +1 ) = 0\) = (5 cos \(\theta\) - 3 ) (cos \(\theta\) + 1) = 0 = 5 cos \(\theta\) - 3 = 0 or cos \(\theta\) + 1 = 0 = cos \(\theta\) = \(\frac 35\) or cos \(\theta\) = -1 cos \(\theta\) \(\neq\) -1 \(\because\) If cos \(\theta\) = -1 then sin \(\theta\) = \(\sqrt {1-cos^2\theta} = \sqrt {1-(-1)^2} = \sqrt {1-1} = 0\) Then cosec \(\theta\) = \(\frac 1{sin \theta} = \frac 10 = \infty\) (Not defined) \(\therefore\) cos \(\theta\) = \(\frac 35\) Correct option is: A) \(\frac{3}{5}\) |
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