Introduction to Trigonometry
Trigonometry (from Greek trigonon "triangle" + metron "measure")
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Here is a quick summary.
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Trigonometry ... is all about triangles. 
Right Angled Triangle
A rightangled triangle (the right angle is shown by the little box in the corner) has names for each side:
 Adjacent is adjacent to the angle "θ",
 Opposite is opposite the angle, and
 the longest side is the Hypotenuse.
Angles
Angles (such as the angle "θ" above) can be in Degrees or Radians. Here are some examples:
Angle  Degrees  Radians 

Right Angle  90°  π/2 
__ Straight Angle  180°  π 
Full Rotation  360°  2π 
"Sine, Cosine and Tangent"
The three most common functions in trigonometry are Sine, Cosine and Tangent. We will use them a lot!
They are simply one side of a triangle divided by another.
For any angle "θ":

Example: What is the sine of 35°?
Using this triangle (lengths are only to one decimal place): sin(35°) = Opposite / Hypotenuse = 2.8/4.9 = 0.57... 
Sine, Cosine and Tangent are often abbreviated to sin, cos and tan.

Try It!Have a try! Move the mouse around to see how different angles affect sine, cosine and tangent And you will also see why trigonometry is also about circles! Notice that the sides can be positive or negative according to the rules of Cartesian coordinates. This makes the sine, cosine and tangent vary between positive and negative also. 
Unit Circle
What you just played with is the Unit Circle.
It is a circle with a radius of 1 with its center at 0.
Because the radius is 1, we can directly measure sine, cosine and tangent.
Here we see the sine function being made by the unit circle:
You can also see the nice graphs made by sine, cosine and tangent.
Repeating Pattern
Because the angle is rotating around and around the circle the Sine, Cosine and Tangent functions repeat once every full rotation.
When we need to calculate the function for an angle larger than a full rotation of 2π (360°) we subtract as many full rotations as needed to bring it back below 2π (360°):
Example: what is the cosine of 370°?
370° is greater than 360° so let us subtract 360°
370° − 360° = 10°
cos(370°) = cos(10°) = 0.985 (to 3 decimal places)
And when the angle is less than zero, just add full rotations.
Example: what is the sine of −3 radians?
−3 is less than 0 so let us add 2π radians
−3 + 2π = −3 + 6.283 = 3.283 radians
sin(−3) = sin(3.283) = −0.141 (to 3 decimal places)
Solving Triangles
A big part of Trigonometry is Solving Triangles. "Solving" means finding missing sides and angles.
Example: Find the Missing Angle "C"
Angle C can be found using angles of a triangle add to 180°:
So C = 180° − 76° − 34° = 70°
It is also possible to find missing side lengths and more. The general rule is:
When we know any 3 of the sides or angles we can find the other 3
(except for the three angles case)
See Solving Triangles for more details.
Other Functions (Cotangent, Secant, Cosecant)
Similar to Sine, Cosine and Tangent, there are three other trigonometric functions which are made by dividing one side by another:

Trigonometric and Triangle Identities
The Trigonometric Identities are equations that are true for all rightangled triangles. 

The Triangle Identities are equations that are true for all triangles (they don't have to have a right angle). 
Enjoy becoming a triangle (and circle) expert!