Lesson-4.1.15- alculate unknown angles and give explanations using the following geometrical properties of circles: ??? angle in a semicircle = 90?? ??? angle between tangent and radius = 90?? ??? angle at the centre is twice the angle at the circumference ??? angles in the same segment are equal ??? opposite angles of a cyclic quadrilateral sum to 180?? (supplementary) ??? alternate segment theorem.