Pythagoras

     Is the result of the process taken as a standard or not.

     
“These two triangles by ¿this¿ nature give the rectangle”.

This aspect might never have struck you.

10
a + (<…>) = (<…>+ b) +c
a+(b+1) = (a+b) +1
a + (b+2) = (a+b)+2
     It seems you can't get out. You must adopt a+(b+c) = (a+b)+c if you adopt a+(b+1) = (a+b) +1.