Math CentralQuandaries & Queries


Question from shanaia, a student:

given that sin A=4/5 and A is obtuse. find sin (A/2)

Hi Shanaia,

Use the fact that $\sin^2 A + \cos^2 A = 1$ to find $\cos A.$ How do you know which sign to use?

Now use the fact that $\cos 2B = \cos^2 B - \sin^2 B = 1 - 2 \sin^2 B$ with $ B = \large \frac{A}{2}.$


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