# One more quick question!

• Oct 28th 2006, 08:02 AM
classicstrings
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)?
• Oct 28th 2006, 08:07 AM
CaptainBlack
Quote:

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
• Oct 28th 2006, 08:19 AM
classicstrings
Ok, is this the same as sin? (tan?) Thanks
• Oct 28th 2006, 08:35 AM
CaptainBlack
Quote:

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
• Oct 28th 2006, 08:37 AM
Aglaia
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