Posted by **Ashley** on Saturday, April 14, 2007 at 1:58pm.

If cosx = 10/19 an pi < x < 2pi, find the exact value of cos x/2

Use the double angle formula.

Cos 2Y= sin^2 Y - cos^2Y = 1-2Cos^2 Y.

let y= x/2, and 2Y=x

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