It can be proven easily that the sum of the three angles of a D is 180⁰

Figure 2.2

D ABC in figure 2.2 is a triangle with line l parallel to seg. BC and passing through A. seg. AB is a transversal on two parallel lines seg. PQ and seg.BC. Hence m Ð PAB and m ÐABC are equal as they are alternate interior angles. Similarly m Ð QAC = m Ð ACB.

Now Ð PAQ = m Ð PAB + m Ð BAC + m Ð CAQ

i.e. 180⁰ = m Ð ABC + m Ð BAC + m Ð ACB

m Ð PAQ = 180⁰ because it is a straight line. Thus the sum, of the measures of the three angles, of any triangle, is 180⁰.

Index

2.1 Introduction
2.2 Sum Of The Angles Of A Triangle
2.3 Types of Triangles
2.4 Altitude, Median And Angle Bisector
2.5 Congruence Of Triangles
2.6 Sides Opposite Congruent Angles

Chapter 3