anonymous
  • anonymous
Evaluate the trig expression cos120 degrees
Mathematics
  • Stacey Warren - Expert brainly.com
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SOLVED
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katieb
  • katieb
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anonymous
  • anonymous
\[\cos \left( 180-\theta \right)=-\cos \theta \]
jim_thompson5910
  • jim_thompson5910
hint: 120 degrees is in quadrant 2 (Q2) the reference angle formula for Q2 is R = 180-x where x is the given angle and R is the reference angle
anonymous
  • anonymous
so 180-120?

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jim_thompson5910
  • jim_thompson5910
yeah so the reference angle is 60 degrees
anonymous
  • anonymous
Is that it?
jim_thompson5910
  • jim_thompson5910
now evaluate cos(60)
anonymous
  • anonymous
-0.95
jim_thompson5910
  • jim_thompson5910
cos(120) = -cos(60) keep in mind cosine is negative in Q2
jim_thompson5910
  • jim_thompson5910
make sure you are in degree mode
anonymous
  • anonymous
-0.0166?
jim_thompson5910
  • jim_thompson5910
no
anonymous
  • anonymous
Sorry if it's coming out weird. I don't have a scientific calculator (it broke) so I'm using the google calculator
jim_thompson5910
  • jim_thompson5910
type in "cos(60 degrees)" without quotes
anonymous
  • anonymous
0.5!
jim_thompson5910
  • jim_thompson5910
google defaults in radian mode so you have to explicitly tell it to be in degree mode
anonymous
  • anonymous
\[\cos 60=\frac{ 1 }{ 2 }\]
jim_thompson5910
  • jim_thompson5910
so your final answer should be -1/2
anonymous
  • anonymous
Ok :) Thanks!
jim_thompson5910
  • jim_thompson5910
yw

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