Given: , x lies in 3rd quadrant
Sign Convention: For knowing the sign of Identities we will take the Graph Shown below which shows in which Quadrant which Trigonometric Ratio is Positive.
We know the identity:
sin 2 x + cos 2 x = 1Putting the value of cos x from the question we get,
(∵ x lies in 3rd quadrant)
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