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Home > A2C > Chapter 9 > 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.