The most wellknown theorem in Euclid Geometry is about the rightangle
triangle, commonly attributed to Pythagoras.
Chinese mathematicians have known about this for a long time. A proof
of the theorem is given in a mathematics book called
"Zhoubi Suanjian" , as well as in a textbook called
"Mathematics in Nine
Chapters" , both of which have never been out of print in China.
Theorem states that: a^{2} + b^{2} = c^{2}
where a and b are the two sides and c is the hypotenus of a rightangle triangle.



Proof as given in Zhoubi Suanjian , Chap. 1.
Draw a rectangle with two sides of a and b(say the lower left).
Draw a diagonal of this rectangle (call it c).
Draw a square using the diagonal as its side (shown in red).
Replicate the original rectangle (marked by the red dot) three times around the red square.
Area of red square = Areas of 4 triangles + Area of small square.
the hypotenus;
This is easily reduced to:
(Let a and b be the two sides and c the diagonal)
4 (a b / 2) + ( b  a )^{2} = c ^{2}
2 a b + b^{2}  2 a b + a ^{2} = c ^{2}
a^{2} + b^{2} = c^{2}
Note: The diagram on the right is a reprint from an old edition with color added.
The diagram on the left is a copy of the original diagram with the grids removed.



