This is a list of the world's railway operating companies listed alphabetically by continent and country. This list includes companies both operating now and in the past.
... common denominator is 4a2. We get
<math>\left(x+\frac{b}{2a}\right)^2=\frac{-4ac+b^2}{4a^2}=\frac{b^2-4ac}{4a^2}.</math>
Taking square roots of both ...