The exact value of cos(-5pi/3) can be written in the form ((a sqr root b)/c) .
Determine the values of a, b and c.

I know that the angle is -300 therefore the reference angle is 60 in the first quadrant. Also, by looking at the special angles I know that b and c are 1/2, but will 'a' be positive or negative ?

Question

The exact value of cos(-5pi/3) can be written in the form ((a sqr root b)/c) . 
Determine the values of a, b and c. 

I know that the angle is -300 therefore the reference angle is 60 in the first quadrant. Also, by looking at the special angles I know that b and c are 1/2, but will 'a' be positive or negative ?

anonymous · Answer

cos(-5pi/3) = -cos(5pi/3) = -cos(2 pi - pi/3) = -cos(pi /3) =- cos 60 =- 1/2 
a=-1 b=1 c=2

savvy · Answer

dude cos(-x)=cos(x)
hence $$\cos(-5\pi/3)=\cos(5\pi/3)$$
$$= \cos(2\pi - \pi/3)$$
$$=\cos(\pi/3)$$
=1/2
hence, a and $$\sqrt{b}$$ could take any values such that their product is 1. c=2

anonymous · Answer

Should be  $$\cos (2\pi-5\pi/3)$$

savvy · Answer

where...????

savvy · Answer

no its in general...

anonymous · Answer

Ok

anonymous · Answer

sorry for that. I just though it in a wrong way