Lesson 9, Topic 1
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Angles in a Triangle

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Tri-angle means three angles. 

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A triangle is the simplest form of polygon with three sides and three interior angles. 

Sum of angles in a triangle is 180º

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\( \scriptsize \angle A \: + \: \angle B \: + \: \angle C = 180^o \)

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 From the above diagram above 

a = y (alternate angles

b = z (alternate angles

but, a + x + b = 180º (sum of angles on a straight line). 

Since a + b = y + z 

Hence x + y + z = a + x + b = 180º

Therefore, the sum of angles of a triangle = 180º


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