1. CHAPTER 21 Triangles

2. Types of Triangles Triangles with THREE ACUTE ANGLES are called ACUTE ANGLED triangles. Triangles with ONE OBTUSE ANGLE are called OBTUSE ANGLED triangles. The sum of the INTERIOR ANGLES of a triangle is 180° 92°+37°=129° z = 180° - 129° z = 51° 37° z 92°

3. Exterior Angle of a Triangle The EXTERIOR ANGLES of a triangle is always equal to the sum of the two opposite INTERIOR ANGLES. x = 46° + 73° x = 119° NOTE ∆ means triangleext means EXTERIOR ANGLE means Angleint means INTERIOR ANGLE supp means SUPPLEMENTARY ANGLE 37° x 92°

4. Special Triangles 1. Scalene a) All sides different lengths b) All angles different sizes c) No axes of symmetry 2. Isosceles a) Two sides the same length b)Two angles the same size c) One axis of symmetry 3. Equitlateral a) Three sides the same length b) Three angles the same size (60°) c) Three axes of symmetry d) Order of rotational symmetry = 3

5. Drawing Triangles If you are just given the lengths of the 3 sides of the triangle you must use a pair of compasses and a ruler. If you are given the length of two sides and the size of the angle between them you must use a protractor and a ruler. Remember : always draw in pencil The PERIMETER of a triangle is the total length of its 3 sides.

6. Area of a Triangle The area of a TRIANGLE is calculated using the formula: Area = ½ x base x perpendicular height this is sometimes written as: Area = base x perpendicular height 2 A = ½ x b x h A = ½ x 8 x 6 A = ½ x 48 A = 24cm 2 8cm 6cm