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Prove the following : cos A + cos (120...

Prove the following :
`cos A + cos (120^@ - A) + cos (120^@+ A) = 0`.

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MODERN PUBLICATION-TRIGNOMETRY-EXERCISE
  1. Prove the following : cos 7 theta + cos 5 theta + cos 3 theta +cos t...

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  2. Prove the following : sin alpha+ sin (alpha+ (2 pi)/3)+ sin (alpha+ ...

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  3. Prove the following : cos A + cos (120^@ - A) + cos (120^@+ A) = 0.

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  4. Prove the following : (sin11 A sin A +sin 7A sin 3A)/ (cos11A sin A ...

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  5. Prove the following: (cos4x + cos3x + cos2x)/(sin4x + sin 3x + sin 2x)...

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  6. Prove the following : (sin 5x -2sin 3x+sinx)/(cos 5x-cos x)= tan x.

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  7. Prove that: ((sin 7x + sin 5x) + (sin 9x + sin 3x))/((cos7x + cos5x) +...

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  8. Prove the following: (sin x -sin 3x)/(sin^2 x -cos^2 x)= 2sinx

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  9. Prove that: (cos x + cos y)^2 + (sin x -sin y)^2 = 4 cos^2frac (x+y)(2...

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  10. Prove that: (cosx-cosy)^2 +(sinx-siny)^2=4 sin^2frac (x-y)(2)

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  11. Prove the following: cot 4x (sin 5x + sin 3x) = cot x (sin 5x -sin 3x)

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  12. If alpha+beta=pi/2, then the maximum value of cosalpha cos beta is:

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  13. If cos (theta + 2alpha) = m cos theta, prove that : cot alpha = (1+m)/...

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  14. If (sin beta)/(sin(2 alpha+ beta))=m/n, prove that : cot (alpha+ beta)...

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  15. If (sin (theta+alpha))/(cos (theta-alpha))= (1-m)/(1+m), prove that : ...

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  16. Prove that : sin (y + z-x) + sin (z + x-y) + sin (x +y-z)-sin (x+y+z)...

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  17. If cos (A + B) sin (C- D) = cos (A- B) sin (C+D), show that : tan A t...

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  18. Prove that : (sinx)/(1+cos x)= tan frac (x)(2).

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  19. Prove that : cot 2A= (cot^2 A-1)/(2 cot A).

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  20. Prove that : (1-sin2x)/(1+sin2x)= tan^2 (pi/4-x).

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