AB is a diameter of a circle centred at the origin O, and P is any point on the circumference of the circle.

Using the position vectors of A, B and P, prove (using a scalar product) that AP is perpendicular to BP (i.e. the angle in the semicircle is a right angle).

Not sure where to even start. I know that the product of vectors has to equal 0 for them the be perpendicular, but I dont know the magnitude for each vector, so how can you prove they are perpendicular?

Any help appreciated.

thank you