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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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AAKASH SERIES-TRANSFORMATIONS AND INDENTITIES-EXERCISE -II
  1. If 2cosx+2cos3x=cosy and 2sinx+2sin3x=siny then cos 2x=

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  2. (1+cos 56^(@)+cos58^(@)-cos66^(@))/(cos28^(@)cos29^(@)sin33^(@))=kcos(...

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  3. (cos20^(@)+8sin10^(@)sin50^(@)sin70^(@))/(sin^(2)80^(@))=

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  4. (sin 5 alpha-sin3alpha)/(cos 5 alpha+2 cos 4 alpha+cos 3alpha)=

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  5. If cos A=3/4 then 32 "sin"A/2."sin"(5A)/2=

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  6. ((cosalpha-cos 3 alpha)(sin8alpha+sin 2alpha))/((sin 5alpha-sin alpha)...

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  7. If 3 sin alpha=5sin beta then (tan((alpha+beta)/2))/(tan((alpha-beta)/...

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  8. m tan (theta-30)=n tan (theta+120) then (m+n)/(m-n)=

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  9. If x/y=(cos A)/(cos B) then (xtannA+ytanB)/(x+y) is equal to

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  10. If (cos(theta(1)-theta(2)))/(cos(theta(1)+theta(2)))+(cos (theta(3)+th...

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  11. If cos 2B=(cos (A+C))/(cos(A-C)) , then tan A , tan B , tan C are in

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  12. sin alpha=sinbeta, cos alpha=cos beta then

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  13. The vlaue of sinx+siny=a and cos x+cosy=b then cos(x-y)=

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  14. (sin 7 theta+ 6 sin 5 theta+17sin 3 theta+12sin theta)/(sin 6 theta+5 ...

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  15. If alpha+beta+gamma=2 theta then cos theta+cos(theta-alpha)+cos (the...

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  16. If A+B+C=2S then cos^(2)S+cos^(2)(S-A)+cos^(2)(S-B)+cos^(2)(S-C)=

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  17. If A+B+C=180^(@), then sin 2 A+sin 2B-sin2C=

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  18. If A+B+C=90^(@) then cos^(2)A+cos^(2)B+cos^(2)C=

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  19. If A+B+C=90^(@) then sin^(2)A+sin^(2)B+sin^(2)C=

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  20. If A+B+C=90^(@) then cos^(2)A+cos^(2)B+cos^(2)C=

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