Trigonometric identities are mathematical identities that involve trigonometric functions. Wow... that was deep. What this means is that for any values for each variable you enter into the equation, the equation is either true or undefined.

The half angle identities (ever wanted to find sin(pi/12) ?) are as follows:

_____________
/ 1 - cos x
sin(x/2)= / ---------
\/ 2
_____________
/ 1 + cos x
cos(x/2)= / ---------
\/ 2
_____________
/ 1 + cos x
tan(x/2)= / ---------
\/ 1 - cos x
_____________
/ 2
csc(x/2)= / ---------
\/ 1 - cos x
_____________
/ 2
sec(x/2)= / ---------
\/ 1 + cos x
_____________
/ 1 - cos x
cot(x/2)= / ---------
\/ 1 + cos x

Also, the addition and double angle identities for tangent are:

tan a + tan b
tan(a+b)= ------------------
1 - (tan a)(tan b)
2 tan a
tan(2a)= ---------
1 - tan^{2}a