Trigonometric Identities
You might like to read our page on Trigonometry first!
Right Triangle
The Trigonometric Identities are equations that are true for Right Angled Triangles ...
... if it is not a Right Angled Triangle refer to our Triangle Identities page.
Each side of a right triangle has a name:
(Adjacent is adjacent to the angle, and Opposite is opposite ... of course!)
Important: We are soon going to be playing with all sorts of functions and it can get quite complex, but remember it all comes back to that simple triangle with:
 Angle θ
 Hypotenuse
 Adjacent
 Opposite
Sine, Cosine and Tangent
The three main functions in trigonometry are Sine, Cosine and Tangent.
They are just the length of one side divided by another
For a right triangle with an angle θ :

Also, if we divide Sine by Cosine we get:
So we can also say:
tan(θ) = sin(θ)/cos(θ) 
That is our first Trigonometric Identity.
But Wait ... There is More!
We can also divide "the other way around" (such as Adjacent/Opposite instead of Opposite/Adjacent):

Example: if Opposite = 2 and Hypotenuse = 4 then
sin(θ) = 2/4, and csc(θ) = 4/2
Because of all that we can say:
sin(θ) = 1/csc(θ)  cos(θ) = 1/sec(θ)  tan(θ) = 1/cot(θ) 
And the other way around:
csc(θ) = 1/sin(θ)  sec(θ) = 1/cos(θ)  cot(θ) = 1/tan(θ) 
And we also have:
cot(θ) = cos(θ)/sin(θ) 
Pythagoras Theorem
For the next trigonometric identities we start with Pythagoras' Theorem:
The Pythagorean Theorem states that, in a right triangle,the square of a (a²) plus the square of b (b²) is equal to the square of c (c²): a^{2} + b^{2} = c^{2} 
Dividing through by c^{2} gives
a^{2}  +  b^{2}  =  c^{2} 
c^{2}  c^{2}  c^{2} 
This can be simplified to:
Now, a/c is Opposite / Hypotenuse, which is sin(θ)
And b/c is Adjacent / Hypotenuse, which is cos(θ)
So (a/c)^{2} + (b/c)^{2} = 1 can also be written:
sin^{2 }θ + cos^{2} θ = 1 
Note: writing sin^{2} θ means to find the sine of θ, then square it.
If I had written sin θ^{2} I would have meant "square θ, then do the sine function"^{}
Example: when the angle θ is 1 radian (approximately 57°):
sin(θ) = 84.1/100 = 0.841
cos(θ) = 54.0/100 = 0.540
0.841^{2} + 0.540^{2} = 0.707 + 0.292 = 0.999
(Close enough to 1, considering we only used 3 decimal places)
Related identities include:

How Do You Remember Them?The identities mentioned so far can be remembered

More Identitites
There are many more identities ... here are some of the more useful ones:
Opposite Angle Identities
sin (θ) =  sin (θ)  cos (θ) = cos (θ)  tan (θ) =  tan (θ) 
Double Angle Identities
Half Angle Identities
Note that "±" means it may be either one, depending on the value of θ/2
Angle Sum and Difference Identities
Note that means you can use plus or minus, and the means to use the opposite sign.
Triangle Identities
There are also Triangle Identities which apply to all triangles (not just Right Angled Triangles)