If AB is the diameter of a circle and P another point on the circumference then Euclidean Geometry tells us that angle APB=90 degrees. Use this fact to show that the equation of the circle whose diameter has endpoints
A(x1,x2) and B(x2,y2) is (x-x1)(x-x2)+(y-y1)(y-y2)=0
(all the numbers above should be sub script, sorry)
Any help greatly appreciated.