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anonymous

  • one year ago

Evaluate the trig expression cos120 degrees

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  1. anonymous
    • one year ago
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    \[\cos \left( 180-\theta \right)=-\cos \theta \]

  2. jim_thompson5910
    • one year ago
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    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
    • one year ago
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    so 180-120?

  4. jim_thompson5910
    • one year ago
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    yeah so the reference angle is 60 degrees

  5. anonymous
    • one year ago
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    Is that it?

  6. jim_thompson5910
    • one year ago
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    now evaluate cos(60)

  7. anonymous
    • one year ago
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    -0.95

  8. jim_thompson5910
    • one year ago
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    cos(120) = -cos(60) keep in mind cosine is negative in Q2

  9. jim_thompson5910
    • one year ago
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    make sure you are in degree mode

  10. anonymous
    • one year ago
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    -0.0166?

  11. jim_thompson5910
    • one year ago
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    no

  12. anonymous
    • one year ago
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    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
    • one year ago
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    type in "cos(60 degrees)" without quotes

  14. anonymous
    • one year ago
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    0.5!

  15. jim_thompson5910
    • one year ago
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    google defaults in radian mode so you have to explicitly tell it to be in degree mode

  16. anonymous
    • one year ago
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    \[\cos 60=\frac{ 1 }{ 2 }\]

  17. jim_thompson5910
    • one year ago
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    so your final answer should be -1/2

  18. anonymous
    • one year ago
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    Ok :) Thanks!

  19. jim_thompson5910
    • one year ago
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    yw

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