Why is the cosine of an obtuse angle negative?

1 Answer
Nov 22, 2015

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


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

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