Trigonometric Identities

You might like to read about Trigonometry first!

Right Triangle

The Trigonometric Identities are equations that are true for Right Angled Triangles.

(If it isn't a Right Angled Triangle use the Triangle Identities page)

Each side of a right triangle has a name:

Adjacent is always next to the angle

And Opposite is opposite the angle

We will soon play with all sorts of functions, but remember it all comes back to that simple triangle with:

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 :

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

For a given angle θ each ratio stays the same
no matter how big or small the triangle is

More About Tan

When we divide Sine by Cosine we get:

sin(θ)cos(θ) = Opposite/HypotenuseAdjacent/Hypotenuse = OppositeAdjacent = tan(θ)

tan(θ) = sin(θ)cos(θ)

That is our first Trigonometric Identity.

Cosecant, Secant and Cotangent

We can also divide "the other way around" (such as Adjacent/Opposite instead of Opposite/Adjacent) to get:

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

Example: when Opposite = 2 and Hypotenuse = 4 then

sin(θ) = 2/4, and csc(θ) = 4/2

Because of all that we can say:

And the other way around:

And we also have:

Heaps more Trigonometric Identities for you!

Pythagoras Theorem

For the next trigonometric identities we start with Pythagoras' Theorem:

The Pythagorean Theorem says that, in a right triangle, the square of a plus the square of b is equal to the square of c:

Dividing through by c 2 gives

a 2 c 2 + b 2 c 2 = c 2 c 2

This can be simplified to:

( ac ) 2 + ( bc ) 2 = 1

a/c is Opposite / Hypotenuse, which is sin(θ)
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

sin 2 θ means to find the sine of θ, then square the result, but
sin θ 2 means to square θ, then do the sine function

Example: 32°

Using 4 decimal places only:

sin(32°) = 0.5299.
cos(32°) = 0.8480.

Now let's calculate sin 2 θ + cos 2 θ:

0.5299 2 + 0.8480 2
= 0.2808. + 0.7191.
= 0.9999.

We get very close to 1 using only 4 decimal places. Try it on your calculator, you might get better results!

Related identities include:

sin 2 θ = 1 − cos 2 θ
cos 2 θ = 1 − sin 2 θ
tan 2 θ + 1 = sec 2 θ
tan 2 θ = sec 2 θ − 1
cot 2 θ + 1 = csc 2 θ
cot 2 θ = csc 2 θ − 1

How Do You Remember Them?

The identities mentioned so far can beremembered
using one clever diagram called the Magic Hexagon:

But Wait . There is More!

There are many more identities . here are some of the more useful ones: