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| Theorem 5 |
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| The sum of the three angles of a triangle is 180o. |
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| A triangle MNS. |
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| MN is a transversal. |
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 (Alternate angles) |
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  . …(2) (Alternate angles) |
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| From the figure, |
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 (  is a straight line and sum of the angles at M = 180o) |
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| From (1) and (2), |
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| Thus it is proved that sum of the measures of the three angles of a triangle is equal to 180o or two right angles. |
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Lines Angles and Triangles |
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