Posted by **Joe** on Wednesday, July 13, 2011 at 4:10pm.

Given that SIN Theta = -1/sqrt(10) and theta is in quadrant 4, find sin2theta and cos2theta.

I'm not sure how to properly square the -1/sqrt10 and keep an exact answer other than decimal. Otherwise, my final answer is Sin2Theta = -.7255970691 and cos2theta = 1.632455533

- trig -
**Reiny**, Wednesday, July 13, 2011 at 7:08pm
sketch a triangle in quadrant IV with opposite side 1 unit long, and the hypotenuse √10 units long.

The the adjacent side is 3 units

sinØ = -1/√10

cosØ = 3/√10 , remember the cosine is + in IV

sin 2Ø = 2sinØcosØ = 2(-1/√10)(3/√10) = -6/10 = -3/5

cos 2Ø = cos^2 Ø - sin^2Ø = 9/10 - 1/10 = 4/5

(I knew your answer was not correct since the cosine or sine of any angle cannot be bigger than 1. )

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