# How do you calculate #cos^-1 (-0.8)#?

##### 1 Answer

Dec 16, 2015

#### Explanation:

Since -0.8 is not a value you would find in a trigonometric ratio, which are:

#costheta=-0.8#

#color(white)(xx)theta=cos^-1(-0.8)#

#color(white)(xxx)~~143.13^@#

However, according to the CAST rule, cosine is also negative in the third quadrant, which means there is a second possible answer.

First, find the related acute angle, and add the angle to

**Finding the related acute angle**

**Finding the second possible angle**