Real Life Applications of Complex or Imaginary Numbers

When solving a quadratic equation using the quadratic formula, it is possible for the b2 - 4ac term inside the square root (the discriminant) to be negative, thus forcing us to take the square root of a negative number. The solutions to the equation will then be complex numbers (i.e., involve the imaginary unit i).

Question:
In the real world, where might these so-called imaginary numbers be used?

When using a formula, we often know the value of one variable to a greater degree of accuracy than we know the others. I need help to understand, what affect, if any, does it make on our use of a formula if we know the value of one variable to a greater degree of accuracy than another? Please assist.