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count, $cos\alpha$ ,when $sin2\alpha =5/13$ ja $-\frac\pi4\leq\alpha \geq\frac\pi4$

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Guest Feb 14, 2018

edited by Guest Feb 14, 2018
edited by Guest Feb 14, 2018

ElectricPavlov · Answer 1 · 2018-02-14T02:23:13

.98058 You DO know there is a calculator on this website......Yes?

CPhill · Answer 2 · 2018-02-14T03:42:01

Sin (2A) = 5/13 ...... so ....Cos(2A) = 12/13

Using a half-angle identity

cos (A) = √ [ ( 1 + cos2A) / 2 ] = √ [ ( 1 + 12/13) / 2 ] =

√ [ (13 + 12) / 26 } = √ [ 25 / 26 ] = 5 / √26 =

(5/26)√26

And we take the positive root since the cosine is positive on the requested interval