1. Geometric Proof GCSE Higher

2. Prove that the sum of the angles in a triangle is 180° A C B We can use previous knowledge of the properties of parallel lines EXTEND the line AB to a point G G DRAW line BE parallel to AC E

3. Prove that the sum of the angles in a triangle is 180° A C BG E <ACB = <CBEAlternate Angles <CAB = <EBGCorresponding Angles <CBA + <CBE + <EBG = 180º Angles on a straight line SO <CBA + <ACB + <CAB = 180º The Angle Sum of a triangle is 180º

4. Using this … …could you prove that the exterior angle of a triangle is equal to the sum of the two opposite interior angles?