If cosec θ + cot θ = 2, then cos θ =
A) 3/5
B) 4/5
C) 5/3
D) 6/5
Lost your password? Please enter your email address. You will receive a link and will create a new password via email.
Please briefly explain why you feel this question should be reported.
Please briefly explain why you feel this answer should be reported.
Please briefly explain why you feel this user should be reported.
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) \(\frac{3}{5}\)