Find all the roots of the equation x4 - 2x3 + 14x2 - 18x + 45 = 0 given that 1 + 2i is one of its roots.

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Knowing that 1+2i is a root, you also know that 1-2i is a root, so one quadratic factor is
   (x -1)² -(2i)² = x^2 -2x +5

Long division of the given polynomial by this quadratic gives a quotient of 
  x² +9
which has roots ±3i.

Then all the roots are {-3i, 3i, 1-2i, 1+2i}.