### Home > A2C > Chapter Ch9 > Lesson 9.3.2 > Problem 9-155

9-155.

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

*x* = 120° or 240°

Now convert the angles into radians.

See part (a).

If sin(*x*) = 0, y = 0

*x* = 0

Remember the side ratios of 45° right triangles.