A right-angled triangle has two 'legs', the short sides, and the longest side, called the hypotenuse. The sum of the squares of the legs equals the square of the hypotenuse.

In similar triangles, the side lengths may be different, but the angles remain the same. This means that the ratios of the side lengths will be the same. There are special functions which describe these unchanging ratios of side lengths: sine and cosine.

In a right-angled triangle, the lengths of the sides may be expressed as a trigonometric function of the angles.

- sinθ = b/c
- cosθ = a/c
- tanθ = b/a

Learn to recite:

SOH = __ S__ine is the

CAH = __ C__osine is the

TOA = __ T__an is the

This is a neat way to remember the sin, cos and tan functions:

SOH: sinθ = ${\opposite}/{\hypotenuse} = O/H$

CAH: cosθ = ${\adjacent}/{\hypotenuse} = A/H$

TOA: tanθ = ${\opposite}/{\adjacent} = O/A$

The sine function is often written in the short form 'sin', but is pronounced like 'sign'.

sinθ = ${\opposite}/{\hypotenuse} = O/H$

The cosine function also has a short form 'cos'.

cosθ = ${\adjacent}/{\hypotenuse} = A/H$

The 'tan' function is short for 'tangent', but it is not the same as the tangent of a circle.

tanθ = ${\opposite}/{\adjacent} = O/A$

Some commonly needed values of these functions are:

θ | sine | cosine | tan |
---|---|---|---|

0° | 0 | $$1$$ | $$0$$ |

30° | $${1}/{2}$$ | $${√3}/2$$ | $${1}/{√3}$$ |

45° | $${1}/{√2}$$ | $${1}/{√2}$$ | $$1$$ |

60° | $${√3}/{2}$$ | $${1}/{2}$$ | $${√3}$$ |

90° | $${0}$$ | $${1}$$ | $${-}$$ |

What are the sizes of the other angles in the triangle? One is a right angle, 90°. Since the total of all the angles in a triangle is 180°, B̂ and Â must sum to 90°, or Â = 90° - B̂.

What is sin(90°- B̂)? It is ${\opposite}/{\hypotenuse} = a/c$.

We saw that cosB̂ = $a/c$, so we have discovered that cosB = sin(90°- B̂).

Similarly, sinB = cos(90°- B̂).

We know that if we have an angle and the lengths of two sides to a triangle, we can calculate the third length, using sin, cos, or tan, depending on which sides we know.

But, what if we know two sides, such as $b$ and $c$, and we want to know the angle B̂?

sinB̂ = $a/c$, so B̂= arcsin$(a/c)$.

arcsin is a function which finds the angle B̂ from the value of sinB̂.

Similarly, if you know cosB̂ or tanB̂, you can find B̂ from arccosB̂ or arctanB̂.

You will find these functions on your calculator as $sin^{-1}$, $cos^{-1}$, and $tan^{-1}$.

Content © Andrew Bone. All rights reserved. Created : January 28, 2014 Last updated :November 13, 2015

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