21 4 C1 draws and and considers angles around the point AO OC O (algebraic notation may be used, e.g. angles labelled and x y) C1 uses at to find angle (e.g. or angle ABC x) ADC “angle centre...” (e.g. y) C1 for + = + ABC ADC x y C1 complete proof with all reasons given, e.g. angles at a point add up to 360°, the angle at the centre of a circle is twice the angle at the circumference NB: do not accept angles of a cyclic quadrilateral add up to “opposite 180°”
21 4 C1 draws AO and OC and considers angles around the point O (algebraic notation may be used, e.g. angles labelled x and y) C1 uses “angle at centre...” to find angle ABC (e.g. x) or angle ADC (e.g. y) C1 for ABC + ADC = x + y C1 complete proof with all reasons given, e.g. angles at a point add up to 360°, the angle at the centre of a circle is twice the angle at the circumference NB: do not accept “opposite angles of a cyclic quadrilateral add up to 180°”