1. ## 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)

2. 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
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