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If 2cosx+2cos3x=cosy and 2sinx+2sin3x=si...

If `2cosx+2cos3x=cosy` and `2sinx+2sin3x=siny` then `cos 2x=`

A

`-7//8`

B

`-1//8`

C

`1//8`

D

`7//8`

Text Solution

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The correct Answer is:
A
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Knowledge Check

  • If cos x +cosy+cosalpha=0 , sinx+siny+sinalpha=0 then cot((x+y)/2)=

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    `sinalpha`
    B
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  • If 4cos^(2)x sin x - 2 sin^(2)x=3sinx , then x =

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    B
    `n pi -(-1)^(n)pi//10`
    C
    `2n pi pm pi//10`
    D
    `n pi + pi//10`

  • The number of solutions of the equation 1+cosx+cos2x=sinx+sin2x+sin3x=0 which satisfy the condition (pi)/2 lt |3x-(pi)/2|lepi is

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    1
    B
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    C
    3
    D
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