Question

What are these things called:

-cos x = cos -x

What are the rules to these trig rules?

Thanks

Answer 1

They are called identities. One of the characteristics of identities is that they apply to all angles x.

The above is not a correct identity, since correctly written, it would be:
cos(x) = cos(-x)

Other identities are
sin²(x)+cos²(x)=1
sin(2A)=2sin(A)cos(A)
...

Answer 2

Thanks for the prompt reply. How do these work out? The Odd/Even Identities?

sin -x = -sin x
cos -x = cox x
tan -x = -tanx

Why does this work out? Does it apply to certain quadrants or something?
Thanks.