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Question: Answered & Verified by Expert
If \(\sin \theta=\frac{2 t}{1+t^2}\) and \(\theta\) lies in the second quadrant, then \(\cos \theta\) is equal to
MathematicsTrigonometric Ratios & IdentitiesWBJEEWBJEE 2011
Options:
  • A \(\frac{1-t^2}{1+t^2}\)
  • B \(\frac{t^2-1}{1+t^2}\)
  • C \(\frac{-\left|1-t^2\right|}{1+t^2}\)
  • D \(\frac{1+t^2}{\left|1-t^2\right|}\)
Solution:
2640 Upvotes Verified Answer
The correct answer is: \(\frac{-\left|1-t^2\right|}{1+t^2}\)
Hints: \(\theta\) in 2nd quad \(\cos \theta < 0\)
\(\begin{aligned}
& |\cos \theta|=\left|\frac{1-t^2}{1+t^2}\right|=\frac{\left|1-t^2\right|}{1+t^2} \\
& \cos \theta=-\frac{\left|1-t^2\right|}{1+t^2}
\end{aligned}\)

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