SOLUTION: express cos120 as a function of an angle in quadrant 1

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Question 79068: express cos120 as a function of an angle in quadrant 1
Answer by jim_thompson5910(35256) About Me  (Show Source):
You can put this solution on YOUR website!
To find the angle, lets take the inverse cosine of cos%28120%29 to get
cos%28cos%28120%29%29%5E-1=pi%2F3
So the angle is pi%2F3

Another way to do this is to notice that the given angle is 120 degrees (which is 2pi%2F3 radians) So to find the reference angle, just subtract this angle from pi radians

pi-2pi%2F3=3pi%2F3-2pi%2F3=%283pi-2pi%29%2F3=pi%2F3