Pythagoras' Theorem is an equation used to give us more information about right angled triangles. The theorem itself looks like this: a2+b2=c2 where c = the hypotenuse (the longest side of the triangle, always opposite the right angle) and a and b are the other two sides of the triangle, either side of the right angle. If we don't know the length of one of the sides of a triangle, we can use Pythagoras' Theorem to figure it out! For example, if we know that a = 3cm and b = 4cm, we can find c accordingly: a2+b2=c2 , 32+42=c2 , 9 + 16 = c2 , 25 = c2 , c = 5 We can also use the equation when we don't know the value of a or b. For example: a2+b2=c2 , a2+52=82 , a2+25=64 , a2=64-25 , a2=39 , a= the square root of 39. The theorem is often used in exam questions to either: discover an unknown length of the side of a right angled triangle; to prove whether a triangle is a right angled triangle or not; to find the length of a line on a graph; and to solve geometry problems regarding both 2D and 3D shapes.