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Find the exact value.

 May 10, 2019
 #1
avatar+8852 
+3

(a)

 

\(\sin^2\theta+\cos^2\theta\,=\,1 \)       by the Pythagorean Identity.

 

\((\frac13)^2+\cos^2\theta\,=\,1\)        because we are given that  \(\sin\theta\,=\,\frac13\)

 

\(\frac19+\cos^2\theta\,=\,1\\~\\ \cos^2\theta\,=\,1-\frac19\\~\\ \cos^2\theta\,=\,\frac89\\~\\ \cos\theta\,=\,\pm\sqrt{\frac89}\\~\\ \cos\theta\,=\,\pm\frac{2\sqrt2}{3} \)

 

Since  θ  is in Quadrant II,  cos θ  must be negative. So   \(\cos\theta\,=\,-\frac{2\sqrt2}{3}\)

 

(b)

 

\(\sin(\theta+\frac{\pi}{6})\,=\,\sin\theta\,\cos\frac{\pi}{6}+\cos\theta\,\sin\frac{\pi}{6}\)     by the angle sum formula for sin

 

\(\sin(\theta+\frac{\pi}{6})\,=\,(\frac13)(\frac{\sqrt3}{2})+(-\frac{2\sqrt2}{3})(\frac12)\\~\\ \sin(\theta+\frac{\pi}{6})\,=\,\frac{\sqrt3}{6}-\frac{2\sqrt2}{6}\\~\\ \sin(\theta+\frac{\pi}{6})\,=\,\frac{\sqrt3-2\sqrt2}{6}\)

 

(c)

 

\(\cos(\theta-\frac{\pi}{3})\,=\,\cos\theta\,\cos\frac{\pi}{3}+\sin\theta\,\sin\frac{\pi}{3}\)     by the angle difference formula for cos

 

Now plug in the values for  cos θ,  cos(pi/3) ,  sin θ , and  sin(pi/3)  and simplify. Can you finish this one?

 

(d)

 

To use the angle sum formula for tan, let's first find  tan θ.

 

\(\tan\theta\,=\,\frac{\sin\theta}{\cos\theta}\,=\,\frac{(\frac13)}{(-\frac{2\sqrt2}{3})}\,=\,-\frac{1}{2\sqrt2}\,=\,-\frac{\sqrt2}{4}\)

 

\(\tan(\theta+\frac{\pi}{4})\,=\,\frac{\tan\theta+\tan\frac{\pi}{4}}{1-\tan\theta\,\tan\frac{\pi}{4}}\)     by the angle sum formula for tan

 

\(\tan(\theta+\frac{\pi}{4})\,=\,\frac{(-\frac{\sqrt2}{4})+(1)}{1-(-\frac{\sqrt2}{4})(1)}\\~\\ \tan(\theta+\frac{\pi}{4})\,=\,\frac{4-\sqrt2}{4+\sqrt2}\)

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 May 10, 2019
 #2
avatar+895 
+2

Thanks, man. On d, do you think I need to rationalize the denominator? I know how, but do I need to?

AdamTaurus  May 10, 2019
 #3
avatar+8852 
+2

I don't know either, but I guess it wouldn't hurt. Maybe better to be safe than sorry!

hectictar  May 10, 2019
 #4
avatar+895 
+2

Nice, thanks!

AdamTaurus  May 10, 2019
 #5
avatar+895 
+2

I got the same answer for c as the answer for b. Is that right?

AdamTaurus  May 10, 2019
 #6
avatar+8852 
+2

Yep they're the same!

hectictar  May 10, 2019

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