2.5 Quadratic equations  (Page 4/14)

Using the quadratic formula

The fourth method of solving a quadratic equation    is by using the quadratic formula    , a formula that will solve all quadratic equations. Although the quadratic formula works on any quadratic equation in standard form, it is easy to make errors in substituting the values into the formula. Pay close attention when substituting, and use parentheses when inserting a negative number.

We can derive the quadratic formula by completing the square    . We will assume that the leading coefficient is positive; if it is negative, we can multiply the equation by $\mathrm{-1}$ and obtain a positive a . Given $a{x}^2+bx+c=0,$ $a\ne 0,$ we will complete the square as follows:

1. First, move the constant term to the right side of the equal sign:

   $$a{x}^2+bx=-c$$

2. As we want the leading coefficient to equal 1, divide through by a :

   $$x^2+\frac{b}{a}x=-\frac{c}{a}$$

3. Then, find $\frac{1}{2}$ of the middle term, and add ${\left(\frac{1}{2}\frac{b}{a}\right)}^2=\frac{b^2}{4a^2}$ to both sides of the equal sign:

   $$x^2+\frac{b}{a}x+\frac{b^2}{4a^2}=\frac{b^2}{4a^2}-\frac{c}{a}$$

4. Next, write the left side as a perfect square. Find the common denominator of the right side and write it as a single fraction:

   $${\left(x+\frac{b}{2a}\right)}^2=\frac{b^2-4ac}{4a^2}$$

5. Now, use the square root property, which gives

   $$\begin{array}{ccc}\hfill x+\frac{b}{2a}& =& \pm \sqrt{\frac{b^2-4ac}{4a^2}}\hfill \\ \hfill x+\frac{b}{2a}& =& \frac{\pm \sqrt{b^2-4ac}}{2a}\hfill \end{array}$$

6. Finally, add $-\frac{b}{2a}$ to both sides of the equation and combine the terms on the right side. Thus,

   $$x=\frac{-b\pm \sqrt{b^2-4ac}}{2a}$$