The method of completing square is still a long method for solving the quadratic equation, so to make calculation further short and easier, a formula is developed by the mathematician to solve quadratic equation, called quadratic formula.
            In order to derive quadratic formula, the method of completing square is used. This is given below
            Write the equation in standard form
                               where
            Shift constant term on RHS, we get
                       
            Make the coefficient of  as  divide the equation by
                       
            Adding on both sides, we get
                       
                                   
                                   
            Taking square root both sides, we get
                                   
                                   
                                   
                                   
            This is the quadratic formula.

Example:
            Solve the equation by quadratic formula
                       
Solution:
            Write the equation in standard form
                       
            Let, ,
            Using quadratic formula, we get
                         
                       
                       
                       
Either                  or