The angle at the centre of a circle is twice the angle at the circumference of the same arc.[br][br]In plain terms:[br][br]2( [math]\angle[/math][math]\alpha[/math] + [math]\angle[/math][math]\beta[/math] ) = [math]\angle[/math][math]\gamma[/math] + [math]\angle[/math][math]\delta[/math][br][br]Angle [math]\angle[/math][math]\gamma[/math] is twice that of [math]\angle[/math][math]\alpha[/math] and Angle [math]\angle[/math][math]\delta[/math] is twice that of [math]\angle[/math][math]\beta[/math][br][br]The Angle [math]\angle[/math]AOB is twice that of the angle at the circumference from the Arc of A and B.