Evaluate cos 210° without using a calculator by using ratios in a reference triangle.

Is the answer -3sqrt/2?

Question

Evaluate cos 210° without using a calculator by using ratios in a reference triangle.

Is the answer -3sqrt/2?

zepdrix · Answer

210 is a special angle Ummmm lemme think...
So 180 + 30 right?

Sooooo ummm cosine is negative in the third quadrant.
andddd it's umm... the longer of the 2 lengths, so it's not the 1/2

Yah that sounds right! :) Good job!

Although I did it by remembering my special angles.
They want you to use a triangle as reference, do you know how to do that? c:

anonymous · Answer

Yeah, I just wanted to make sure I got the answer right. I'm a little sketchy on this so I wanted to double check. Thanks a ton man.