How do you evaluate sin(arccos (1/3)) sin(arccos(13))?

1 Answer
Jun 10, 2016

sin(arccos(1/3))=+-(2sqrt2)/3sin(arccos(13))=±223

Explanation:

Let arccos(1/3)=thetaarccos(13)=θ. This means

costheta=1/3cosθ=13 and hence

sintheta=sqrt(1-(1/3)^2)=+-sqrt(1-1/9)=+-sqrt(8/9)=+-(2sqrt2)/3sinθ=1(13)2=±119=±89=±223

We are using both plus and minus as if costhetacosθ is in first quadrant, sinthetasinθ could be positive and if costhetacosθ is in fourth quadrant, sinthetasinθ could be negaitive.

As such sin(arccos(1/3))=sintheta=+-(2sqrt2)/3sin(arccos(13))=sinθ=±223