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Introduction to Trigonometry

Trigonometry (from Greek trigonon "triangle" + metron "measure")

Want to Learn Trigonometry? Here are the basics!
Follow the links for more, or go to Trigonometry Index

triangle Trigonometry ... is all about triangles.

Right Angled Triangle

A right-angled triangle (the right angle is shown by the little box in the corner) has names for each side:

  • Adjacent is adjacent to the angle,
  • Opposite is opposite the angle, and
  • the longest side is the Hypotenuse.
triangle showing Opposite, Adjacent and Hypotenuse


Angles (such as the angle "θ" above) can be in Degrees or Radians. Here are some examples:

Angle Degrees Radians
right angleRight Angle  90° π/2
__ Straight Angle 180° π
right angle Full Rotation 360° 2π

"Sine, Cosine and Tangent"

The three most common functions in trigonometry are Sine, Cosine and Tangent. You will use them a lot!

They are simply one side of a triangle divided by another.

For any angle "θ":

Right-Angled Triangle
Sine Function:
sin(θ) = Opposite / Hypotenuse
Cosine Function:
cos(θ) = Adjacent / Hypotenuse
Tangent Function:
tan(θ) = Opposite / Adjacent


Sine, Cosine and Tangent are often abbreivated to sin, cos and tan.

View Larger

Try It!

Have a try! Drag the corner around to see how different angles affect sine, cosine and tangent

And you will also see why trigonometry is 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 we have just been playing with is the Unit Circle.

It is just a circle with a radius of 1 with its center at 0.

Because the radius is 1, it is easy to measure sine, cosine and tangent.

unit circle
sine cosine plot 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 you need to calculate the function for an angle larger than a full rotation of 2π (360°) just subtract as many full rotations as you need 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(10°) = 0.985 (to 3 decimal places)

Likewise if 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(3.283) = -0.141 (to 3 decimal places)

Solving Triangles

A big part of Trigonometry is Solving Triangles. By "solving" I mean finding missing sides and angles.

Example: Find the Missing Angle "C"


It's easy to find angle C by 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:

If you know any 3 of the sides or angles you 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:

Right-Angled Triangle
Cosecant Function:
csc(θ) = Hypotenuse / Opposite
Secant Function:
sec(θ) = Hypotenuse / Adjacent
Cotangent Function:
cot(θ) = Adjacent / Opposite


Trigonometric and Triangle Identities

right angled triangle

The Trigonometric Identities are equations that are true for all right-angled 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 expert!