The mathematician al-Khwarizmi told the nature of the equation

                            - The nature of the equation -

1. If in addition the same number of sides of the equation is the equation satisfied
2. Even without the same number of sides of the equation in the equation satisfied
3. On both sides of the equation is multiplied by the number of such equation is also satisfied
4. If both sides of the equation as a non-zero divided by the number of equations satisfied