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Equation of Circle in Diameter form

Let AB the diameter of the circle with center O where A(x1, y1) and B(x2, y2) are the end-points of the diameter. 
Let P be any point on the circumference of the circle. Then, AP and BP is joined such that triangle ABP is formed as shown in the figure.
Then,
angle APB = 90    [angle at semi-circle is right angle]