## anonymous one year ago Evaluate the trig expression cos120 degrees

1. anonymous

$\cos \left( 180-\theta \right)=-\cos \theta$

2. 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

3. anonymous

so 180-120?

4. jim_thompson5910

yeah so the reference angle is 60 degrees

5. anonymous

Is that it?

6. jim_thompson5910

now evaluate cos(60)

7. anonymous

-0.95

8. jim_thompson5910

cos(120) = -cos(60) keep in mind cosine is negative in Q2

9. jim_thompson5910

make sure you are in degree mode

10. anonymous

-0.0166?

11. jim_thompson5910

no

12. anonymous

Sorry if it's coming out weird. I don't have a scientific calculator (it broke) so I'm using the google calculator

13. jim_thompson5910

type in "cos(60 degrees)" without quotes

14. anonymous

0.5!

15. jim_thompson5910

google defaults in radian mode so you have to explicitly tell it to be in degree mode

16. anonymous

$\cos 60=\frac{ 1 }{ 2 }$

17. jim_thompson5910

18. anonymous

Ok :) Thanks!

19. jim_thompson5910

yw