Q.15.
Kindly refer the link for your query.
https://www.meritnation.com/ask-answer/question/cos2a-1-tana-sin3a-sina-cosa-1-sina-cosa/introduction-to-trigonometry/5773718
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https://www.meritnation.com/ask-answer/question/cos2a-1-tana-sin3a-sina-cosa-1-sina-cosa/introduction-to-trigonometry/5773718
Regards