Question

Why is the cosine of an obtuse angle negative?

Answer 1

Draw the cosine function between 0 and 180 degrees and you will learn why angles `>90^o` are negative

Explanation:

Here is the graph of cosine:

As you can see, once the cosine function is greater than 90 degrees, the value becomes negative .

Between 0 and 90 degrees , cosine is positive .

hope that helped