# How do you evaluate #cos^-1(-1/2)#?

##### 2 Answers

Which is the same as:

#### Explanation:

Let the unknow angle be

So if this was the condition (it is not!) then

But we have

As the hypotenuse is positive then

So

Thus

so

For radian measure

#### Explanation:

We are asked to find which angle has a cos value of

In a rotation of

The first step is to establish the quadrants in which Cos has negative values,

Cos is negative in the

(between

To find the root angle we use

In the second quadrant:

In the third quadrant: