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Prove that the sum of angles of a triangle is 180

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               Let a triangle ABC and draw a line l through A and parallel to the base BC.

               ∵BC∥l and AB and AC are transversals on it.

               ∴∠4=∠2…(i)                     (Alternate angles)

               and ∠5=∠3…(ii)               (Alternate angles)

               Adding equations (i) and (ii), we get

               ∠4+∠5=∠2+∠3

               Add ∠1 on both sides.

               ⇒∠1+∠4+∠5=∠1+∠2+∠3…(iii)

               ∠1+∠4+∠5=180

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            (∵l is a straight line)

               From (iii),

               ∠1+∠2+∠3=180

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               Hence, sum of angles of a triangle is 180

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Hence Proved.

the sum of The triangle is 180