The other angle with the same value of 2pi/3 for the cosine function is given by:
[tex]\frac{2\pi{}}{3}+\frac{\pi{}}{2}=\frac{7\pi{}}{6}[/tex]Because the cosine and sine function repeat the absolute value every pi/2, to localize the sign you can think about the trigonometric circle.
As I stated before the cosine and sine function repeat the absolute value every pi/2, to localize the sign you can think about the trigonometric circle. The values between pi/2 and 3pi/2 are negative and the others are positive.
B part)
To find the two values with the opposite value we need to take pi/2 from our angle, and add pi to find the two values.
[tex]\frac{2\pi}{3}+\pi=\frac{5\pi}{3}[/tex]and
[tex]\frac{2\pi}{3}-\frac{\pi}{2}=\frac{\pi}{6}[/tex]With this we have all the angles.
