Hhtml> The sum of the angles of a triangle is 180 degrees Date: Wed, 18 Feb 1998 23:38:32 -0500
Sender: Quin
Subject: triangle

How do you prove that the sum of the angles of a triangle is 180 degrees? Is there a proof? what is it?

level: secondary or higher

Hi Quin,

Cut two copies of triangle ABC out of paper. Place the original with A at the top and AB, say, on the left; place the first of the copies upside down on the left with its vertex B on the original's A and vice versa. Place the second copy on the right with its C and A on the original's A and C. Note that the properties of parallel lines (alternate interior angles are equal) force the bases of the two copies to line up parallel to the original's base BC. One sees the three angles about A forming a straight angle.


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