# Thread: One more quick question!

1. ## One more quick question!

I was wondering why cos(3pie/4) = cos(-3pie/4)?

I know 3pie/4 is 135 degrees so in 2nd Quad, where only sin is positive.

But how do I know it is equal to cos(-3pie/4)?

2. Originally Posted by classicstrings
I was wondering why cos(3pie/4) = cos(-3pie/4)?

I know 3pie/4 is 135 degrees so in 2nd Quad, where only sin is positive.

But how do I know it is equal to cos(-3pie/4)?
One way is to use the knowlege that cos is a symmetric function, that is
for any real x: cos(x)=cos(-x).

RonL

3. Ok, is this the same as sin? (tan?) Thanks

4. Originally Posted by classicstrings
Ok, is this the same as sin? (tan?) Thanks
sin and tan are anti symmetric, that is for any real x (with the obvious
exclusions for tan when it is undefined):

tan(-x)=-tan(x),

and

sin(-x)=-sin(x).

RonL

5. use this trick: keep in your mind the gognometric circonference; then usinig radians, divide it in the slices you need...you will soon notice that moving clockwise or counterclockwise you'll get to the same point..it's not what you can define a math proof, but it can help you understand...now you should be able to answer to your question about sin and tan