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Math Help - Trig Angles

  1. #1
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    Trig Angles

    Q:
    1. If tanA=1/3 and pi<A<(3pi/2), calculate cos2A.

    2. In what quadrant does the angle 2A lie?

    A:
    1. I don't need help with this one. Answer: cos2A=4/5 and lies in quadrant 3

    2. I need help with this one. Can someone explain to me how I would know which quadrant the angle 2A would be in? How to approach and answer this?

    Thanks!
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  2. #2
    A riddle wrapped in an enigma
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    Quote Originally Posted by skeske1234 View Post
    Q:
    1. If tanA=1/3 and pi<A<(3pi/2), calculate cos2A.

    2. In what quadrant does the angle 2A lie?

    A:
    1. I don't need help with this one. Answer: cos2A=4/5 and lies in quadrant 3
    Hi skeske1234,

    Cos A is negative in the 3rd quadrant and you found Cos 2A = 4/5. Where is Cos A positive? Don't forget your range for angle A.

    2. I need help with this one. Can someone explain to me how I would know which quadrant the angle 2A would be in? How to approach and answer this?

    Thanks!
    Since Tan A = 1/3, that puts A in quadrant I or quadrant III because that's the only 2 quadrants where Tan A is positive.

    ArcTan A is about 18.4 degrees in quadrant I and 198.4 in quadrant III. Cos 2A would either be about 36.8 degrees in quadrant I or about 323.2 degrees in quadrant IV which is out of range.
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