I can't figure it out

  • Thread starter yeyspaz
  • Start date
  • #1
yeyspaz
11
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I have this trig identity problem i need to solve

cos2x=cosx

I have to find all the values of x, and i know the answer is x=0°+120°, but i can figure ou how. I've tried it over and over and my answer turns out to x=0°=180°. Please help, thank you.:uhh:
 

Answers and Replies

  • #2
Tide
Science Advisor
Homework Helper
3,089
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First, 180 is NOT equal to 0. Second, 180 degrees is obviously not a solution (substitiute back into the equation to see this).

HINT: [tex]\cos 2x = \cos^2 x - \sin^2 x[/tex]
 

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