Early on at school you’re told that the sum of angles in a triangle add up to 180o… but how can you be certain of this?

We use this to work out many other properties in shape, space and mathematics, it is important you know that why this is true and to not just take your teacher's word for it!

Proof

Imagine we have 2 parallel lines and we draw two lines going from the same point.

[picture missing]

We know angles on a straight line add up to 180o, this does not have to be proved because it is by definition!

So, A+B+C = 180o

Using alternate angles, as a property of parallel lines, we can label the two other interior angles of the triangle.

[picture missing]

Thus, the interior angles of a triangle sum up to 180o.

This is just one of many ways to prove this.

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