There are three basic formulae, involving the ratio of the lengths of the sides. Given a right angled triangle, we first label the sides for the angle we have:
If we have the angle shown, we label the sides for this angle. Opposite =is the side opposite the angle x Hypotenuse =is the longest side, always opposite the right angle. Adjacent=a is the remaining side/ We are to calculateWe use one of the formulae 

We choose which formula to use depending on which side we gave got and which side we need to find. In this case we haveand need to findso we use the formula 

Example: Now we haveand want to findWe use the formula 

Example: Now we haveand want to findWe use the formula 

Example: We have o and want to find h. We use the formula 
It is important to realise the difference between the formulae
and
For the first we writebut for the second we actually work out