Thales' Theorem

Good morning! Today I am going to speak about a useful theorem for maths classes, but fisrt I believe that it is necesary making a brief introduction of the author.

Thales of Miletus, about 624 BC– about546 BC), was a pre-Socratic Greek philosopher. He was from Miletus in Asia Minor. Many, notably Aristotle, regard him as the first philosopher in the Greek tradition.  His theorem is called Basic Proportionality Theorem or Thales theorem which say:

“If a straight line is drawn parallel to one of the sides of a triangle, then it cuts the other sides of the triangle, or these produced, proportionally; and, if the sides of the triangle, or the sides produced, are cut proportionally, then the line joining the points of section is parallel to the remaining side of the triangle.”

Proof of Thales' Theorem

Here, you can see a video with a graphical example of that with the use of ITC:

As always, here you have a link to explain this theorem to children in a simple way, in this case the video is in Spanish.

I hope it will be useful for you!