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Homework Help: How to prove this trig equation

  1. Oct 26, 2005 #1
    i have a problem with composed with two questions:
    first i havr to prove that
    cos(3x)=cos(x).(4cos^2(x) - 3)
    okay, for this one, I was able to prove it; then the second question came, saying:
    what is sin(pi/10) and cos(pi/10)
    i dont know how to do that, but it should be related to the question before, how!? i don't know!
    anyone with something to say ... ??
  2. jcsd
  3. Oct 26, 2005 #2


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    From the information you have provided it seems like all you need to do is evaluate the sin and cos at [itex] \frac \pi {10} [/itex]
  4. Oct 26, 2005 #3
    but i cant use the calculator... because the second question doesnt say CALCULATE, in fact, it says NOTE from the previous question what sin and cos pi/10 is!!
  5. Oct 26, 2005 #4
    x = [itex]\frac \pi {10} [/itex]
    [itex]3x = \frac \pi {2} - 2x [/itex]
    Last edited: Oct 26, 2005
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