### Home > A2C > Chapter 9 > Lesson 9.3.2 > Problem9-155

9-155.

$\text{cos}\left(\frac{\pi}{6}\right)=\frac{1}{2}$

What angle has the cosine of negative one half? Think of 30°, 60° right triangles.

x = 120° or 240°

$120\left(\frac{\pi}{180}\right)\,\,\,\,\,\,\,240\left(\frac{\pi}{180}\right)$

Now convert the angles into radians.

$\textit{x}=\frac{2\pi}{3},\,\frac{4\pi}{3}$

See part (a).

If sin(x) = 0, y = 0

x = 0

Remember the side ratios of 45° right triangles.