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Math Help - Trig half angles

  1. #1
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    Trig half angles

    My mind is not working to day if SinA=4/5, find sin 1/2A.

    So in the formula I need Cos, Since SinA=4/5 I know CosA=1/5 and that is the number I will use in my problem

    SO Sin1/2A=Sqrt( (1-Cos(1/5))/2) right?


    and if Cosx =-1/2 find sin x/2

    I will say SinA=Sqrt((1--1/2)/2)
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  2. #2
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    Since SinA=4/5 I know CosA=1/5
    If \sin{A} = \frac{4}{5} , then \cos{A} = \frac{3}{5}

    \sin\left(\frac{A}{2}\right) = \pm \sqrt{\frac{1-\cos{A}}{2}}

    \sin\left(\frac{A}{2}\right) = \pm \sqrt{\frac{1-\frac{3}{5}}{2}}

    \sin\left(\frac{A}{2}\right) = \sqrt{\frac{1}{5}}

    since A is a quad I or quad II angle, \sin\left(\frac{A}{2}\right) > 0<br />
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