I don't understand Pythagoras' theorem . What is it and how do you use it?

Ok, so the reason we need the Pythagoras theorem is to work out the length of missing side given we know the other 2 sides in a right-angled triangle. (Would go on to draw the triangle with the known sides and unknown side). The key rule to remember is that 'the hypotenuse squared( c) is equal to the sum of the squares of the other two sides: a² + b² = c². Any questions?Let's proceed to an example. (Would go on to draw an example with numeric values for the sides of the triangle). For instance if shorter sides we 8 and 9. 8²+9²= c² >>>>>>>>>>>>>>>> 145=c²>>>>>>>>>>>>>>>>>145^1/2>>>>>>>>>>>>>>>>>>>>>>>>>>>>12.04159458>>>>>>>>>>>>>>>>12.04 (3sf) I will now show an example where the hypotenuse/longest side is known and we want to work out one of the shorter sides.