Triangle PQR is a right angled triangle. PQ is the hypotenuse and is 5cm long and QR has length 3cm. What is the length of side PR? Show your working.

Pythagoras' theorem is used to calculate the lengths of the sides of a right angled triangle. 

The theorem is as follows : a² = b² + c², where a is the length of the hypotenuse. The hypotenuse is the side that is opposite to the right angle. PQ is the hypotenuse therefore,

5² = 3² + PR²

(Rearrange)

5² - 3² = PR² 

25 - 9 = PR² 

PR²  = 16

Square root PR is 4. Therefore the length of side PR 4cm.