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If 2 cos x +2cos 3x=cosy, 2 sinx+2sin3x=...

If `2 cos x +2cos 3x=cosy, 2 sinx+2sin3x=siny` then the vlaue of cos 2x is

A

`-7//8`

B

`1//8`

C

`-1//8`

D

`7//8`

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a
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MCGROW HILL PUBLICATION-TRIGONOMETRICAL IDENTITIES AND EQUATIONS -EXERCISE (LEVEL 1 Single Correct Answer Type Questions)
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