If you are given the coordinates of point A, B and C, and have been asked to prove that angle ABC is a right angle, you would use the gradients of different points
Firstly, you would work out the gradient of line AB, by using the formula (y2 - y1 / x2 - x1)
You would then do the same for line BC - If the gradient of line AB is the negative reciprocal of line BC, the 2 lines are perpendicular
Perpendicular lines meet at right angles, and angle ABC meets at line AB and BC, which are perpendicular
This means that angle ABC is a right angle
