trig

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I don't understand how to do this type of question? If 9cos^2 theta +3cos theta=0, where 90 degrees <theta<180 degrees, what is the value of theta, correct to the nearest tenth of a degree. Could someone please explain. Thanks

  • trig -

    9cos^2(t)+3cos(t)=0
    subtract 3cos(t) from both sides
    9cos^2(t)=-3cos(t)
    divide both sides by 3cos(t)
    3cos(t)=-1
    cos(t)=-1/3
    t=arccos(-1/3)

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