anonymous
  • anonymous
1+cos(18x)=_____. a) 2sin^2 (18x) b) 2cos^2 (9x) c) 2cos^2 (18x) d) 2sin^2 (9x)
Mathematics
  • Stacey Warren - Expert brainly.com
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SOLVED
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katieb
  • katieb
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anonymous
  • anonymous
@jim_thompson5910
anonymous
  • anonymous
@mathstudent55
anonymous
  • anonymous
hi need help

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anonymous
  • anonymous
yes i need help
anonymous
  • anonymous
i gave you a medal
anonymous
  • anonymous
with what
jim_thompson5910
  • jim_thompson5910
\[\Large 1 + \cos(18x) = 1 + \cos(2*9x)\] \[\Large 1 + \cos(18x) = 1 + 2\cos^2(9x) - 1\] \[\Large 1 + \cos(18x) = 2\cos^2(9x)\] On step 2, I used this identity \[\Large \cos(2x) = \cos^2(x) - 1\] see this link for more trig identities http://tutorial.math.lamar.edu/pdf/Trig_Cheat_Sheet.pdf specifically see page 2 and the "Double Angle Formulas" section
jim_thompson5910
  • jim_thompson5910
oops I made a typo, I meant to say \[\Large \cos(2x) = 2\cos^2(x) - 1\]
anonymous
  • anonymous
how do i use that equation to find the answer
anonymous
  • anonymous
@jim_thompson5910
anonymous
  • anonymous
someone please help
jim_thompson5910
  • jim_thompson5910
I practically gave you the answer. Read through my steps again.
anonymous
  • anonymous
oh duh.
anonymous
  • anonymous
haha
anonymous
  • anonymous
thanks!
jim_thompson5910
  • jim_thompson5910
You use the identity to replace cos(2*9x) with 2*cos^2(9x) - 1 the "9x" is thought of as the theta in \[\Large \cos(2\theta) = 2\cos^2(\theta) - 1\]
jim_thompson5910
  • jim_thompson5910
np

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