notam lungimile cu x , y, z AB =x BC=y CD =z
A ------------B ---------------------------C ------------D
x y z
AD = x +y+z AC = x +y BD = y +x
relatia : x · z + ( x + y +z ) · y = ( x +y ) ·( y +z )
x · z + x·y + y² + z·y = x·y + y² + x·z + y·z adevarat