Euclid's Pythagoras Theorem proof

[zeion]

Homework Statement

Hi. I'm looking at Euclid's proof of the Pythagoras theorem.
After cutting the square of the hypotenuse I don't understand why the area of the left rectangle (of the hypotenuse square) is equal to the area of the left square?

Homework Equations

The Attempt at a Solution

 

[tiny-tim]

hi zeion!

because the triangle is cut into two triangles each similar to the original …

so the base of the rectangle is a times (a/c)