Page 108 - TB_6B

Objective: Learn about the interior and the exterior angles of polygons

Learn about the sum of the interior angles of a triangle.

DE passes through vertex A and is D A E
parallel to BC. B E
C
Use the characteristics of parallel lines. D A
B
B DAB (alternate angles) C
C EAC (alternate angles) = 180

Therefore,
the sum of the interior angles of
triangle ABC is:
mA mB mC
= m A m DAB m EAC
= 180

The sum of the interior angles of a triangle is 180 .

Find m a in each triangle.

a 25 45
60 40 30 a a

27. Interior and Exterior Angles 97