Question

prove that sum of all angle of triangle is 180 degree ​

Answer 1

Answer:

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Answer 2

We draw a line parallel to AB that passes

through point C.

Angles C and C' are vertical angles,

therefore ∠C = ∠C'.

Angles B and B' are corresponding angles,

therefore ∠B = ∠B'.

Angles A and A' are corresponding angles, therefore ∠A = ∠A'.

So, the angle sum ∠A + ∠B + ∠C is equal to the angle sum ∠A' + ∠B' + ∠C'.

The three angles A', B', and C' form together a straight angle

(they are along the line l).

So, their angle sum is 180°. But then the angle sum ∠A + ∠B + ∠C must also be 180°.