Chapter 4.3, Problem 61E

Solution

To identify: The statement is true or false If θ is an angle of a triangle such that $\cos \theta <0$ , then θ is obtuse.

The statement is true.

Given information

The given If θ is an angle of a triangle such that $\cos \theta <0$ , then θ is obtuse.

Explanation:

Consider the given information

Negative value of cosine only occurs in quadrants $1and3$ which require the angle to be obtuse.

So, the statement is true.