2. We can classify triangles by their sides: Scalene – no sides are the same length
3. We can classify triangles by their sides: Isosceles – two sides are the same length (Base angles are also equal)
4. We can classify triangles by their sides: Equilateral – all sides are the same length (All angles are also equal and 60 °)
5. We can classify triangles by their angles: Acute angles – all angles are acute
6. We can classify triangles by their angles: Obtuse angles – one angle is obtuse
7. We can classify triangles by their angles: Right angled – one angle is a right angle
8. PROOF – Angle Sum of a Triangle Construction: Draw a line DE parallel to AC. D E ﮮ ACB = ﮮ CBE = z ° (Alternate angles on AC║DE) z ° ﮮ BAC = ﮮ ABD = y ° (Alternate angles on AC║DE) y ° ﮮ DBA + ﮮ ABC + ﮮ EBC = 180 ° y° + x° + z° = 180° (Angles on a straight line add to 180 ° ) Therefore ﮮ BCA + ﮮ ABC + ﮮ BAC = 180 °
9. PROOF – Exterior Angle of a Triangle Construction: Draw CE parallel to AB. ﮮ BAC = ﮮ ECD = y ° (Corresponding angles on AC║DE) ﮮ ABC = ﮮ BCE = x ° (Alternate angles on AC║DE) ﮮ BCD = ﮮ ABC + ﮮ BAC z° = x° + y° D E z °
