Slide 1

Angles In Triangles Types of Triangles Equilateral Triangle 3 equal sides 3 equal angles. Isosceles triangle 2 equal sides 2 equal angles (base) Scalene triangle 3 unequal sides 3 unequal angles Slide 2 Any triangle containing a 90 o angle is a right-angled triangle An isosceles or a scalene triangle may contain a right angle. Right-angled isosceles triangles. Right-angled scalene triangle. Slide 3 To determine the angle sum of any Triangle Take 3 indentical copies of this triangle like so: 1 3 2 How can we use this to help us? Angles on a straight line add to 180 o These are the same angles as in the triangle! The angle sum of a triangle = 180 0 Slide 4 Calculating unknown Angles Example 1 a 65 o Calculate angle a. Angle a = 180 (90 + 65) = 180 155 = 25 o Example 2 Calculate angles a, b and c a b c Since the triangle is equilateral, angles a, b and c are all 60 o (180/3) Slide 5 Calculating unknown Angles Angle a = 65 o (base angles of an isosceles triangle are equal). Example 3 a 65 o Calculate angle a. b Angle b = 180 (65 + 65) = 180 130 = 50 o Example 4 Calculate angles x and y y 130 o x Slide 6 Calculating unknown Angles Example 5 Calculate angles a and b. b a Example 6 Calculate angle a 15 o 27 o a Angle a = 180 (15 + 27) = 180 42 = 138 o