If 2cos⁡x+2cos⁡3x=cos⁡y,2sin⁡x+2sin⁡3x=sin⁡y then the value of cos⁡2x of

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– 7/8

1/8

– 1/8

7/8

answer is A.

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Detailed Solution

cos⁡y=2×2cos ⁡2xcos ⁡xsin⁡y=2×2sin⁡2xcos⁡x ⇒1=cos2⁡y+sin2⁡y=16cos2⁡xSo cos⁡2x=2cos2⁡x−1=−7/8

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