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The general quadratic equation  
\[ax^2 +bx+c=0\]
  can be solved for  
\[x\]
  using the formula  
\[x= \frac{-b \pm \sqrt{b^2-4ac}}{2a}\]
.
We can derive this formula by completing the square.
Starting from  
\[ax^2 +bx+c=0\]
, multiply by  
\[a\]
.
\[a^2x^2 +abx+ac=0\]

Ignore  
\[ac\]
  for the moment and complete the square for  
\[a^2x^2 +abx\]
.
\[a^2x^2 +abx+ac=((ax+\frac{b}{2})^2 - (\frac{b}{2})^2)+ac=0 \]

Now add  
\[(\frac{b}{2})^2\]
  and subtract  
\[ac\]
.
\[(ax+\frac{b}{2})^2 = (\frac{b}{2})^2-ac =\frac{b^2}{4}-ac= \frac{b^2-4ac}{4}\]

Square root both sides.
\[ax+\frac{b}{2} = \sqrt{(\frac{b^2-4ac}{4})}=\pm \frac{\sqrt{b^2-4ac}}{2}\]

Now subtract  
\[\frac{b}{2}\]
  from both sides.
\[ax =- \frac{b}{2} \sqrt{(\frac{b^2-4ac}{4})}= \frac{-b \pm\sqrt{b^2-4ac}}{2}=\]

Finally divide both sides by  
\[a\]
.
\[x = \frac{-b \pm \sqrt{b^2-4ac}}{2a}\]
.