Trigonometry and method of difference

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SUMMARY

The discussion centers on proving the equation 1-2cosx+2cos2x-2cos3x+....+2cos8x = cos(17/2)x sec(x/2) using trigonometric identities. The initial equation provided is 1+2cosx+2cos2x+2cos3x+....+2cos8x = sin(8+1/2)x cosec(x/2). Participants suggest substituting 2x for x to simplify the equation and recommend subtracting the modified equation from the original to isolate the left-hand side. The conversation emphasizes the importance of careful manipulation of trigonometric identities to arrive at the desired proof.

PREREQUISITES
  • Understanding of trigonometric identities and equations
  • Familiarity with the sine and cosine functions
  • Knowledge of the method of differences in mathematical proofs
  • Experience with algebraic manipulation of equations
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  • Study the method of differences in trigonometric proofs
  • Learn about the properties of sine and cosine functions
  • Explore advanced trigonometric identities and their applications
  • Practice solving similar trigonometric equations for mastery
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Students studying trigonometry, mathematics educators, and anyone interested in advanced mathematical proofs involving trigonometric identities.

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Homework Statement



Given that 1+2cosx+2cos2x+2cos3x + ....+2cosnx = sin(x+1/2)x cosec(x/2)
Prove that 1-2cosx+2cos2x-2cos3x+....+2cos8x = cos(17/2)x sec(x/2)


Homework Equations





The Attempt at a Solution



1+2cosx+2cos2x+2cos3x + ....+2cos8x = sin(8+1/2)x cosec(x/2)

4cosx+4cos3x+4cos5x... = ?

I don't know how to continue. Or is there another way?
 
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1+2cosx+2cos2x+2cos3x + ....+ 2cos8x = ... eqn(1)
Substitute 2x for x, and you can say
1 + 2cos2x + 2 cos4x + 2cos6x + 2cos8x = ... eqn(2)

If you subtract double eqn(2) from eqn(1) you'll as good as have the left hand side. I don't know whether this will get you anywhere.

Also, fix up the x that should be n in your first line.
 

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