Factorize:. x^3+3x-2?
The crucial step is to replace x with (w+1/w), and to solve for w. This leads to a sixth order equation in w, BUT, it's a sixth order equation that's easy to solve. Here are the details:
x^3 - 3 x + 2 =
(w+1/w)^3 - 3(w+1/w) + 2 =
w^3 + 3 w^2 (1/w) + 3 w (1/w)^2 + (1/w)^3 - 3 (w+1/w) + 2 =
w^3 + 3 w + 3 (1/w) + (1/w3) - 3 (w+1/w) + 2 =
w^3 + (1/w^3) + 2.
So x^3-3x+2=0 becomes w^3+1/w^3+2 = 0
This is a sixth order equation in w. The trick to solving it is to realize that it's really a quadratic in w^3. Replacing w^3 with z, we get:
z + 1/z + 2 = 0
ie z^2 + 1 + 2z = 0.
Solving the quadratic using (z^2 + 1 + 2z = (z+1)^2 ) you get z = -1.
So, w^3 = -1.
A solution to this is w = -1, which gives the solution x = w + 1/w = -1+1/-1 = -2.
Polynomial division now gives
x^3-3x+2 can be factored as (x+2)(x^2 - 2x + 1).
x^2 - 2x + 1 = (x-1)^2, so we have that the solutions are x = -2 and 1 (with multiplicity 2).
We could have used other solutions to w3 = -1. The three solutions are:
w = -1, 1/2 + i √3/2, 1/2 - i √3/2
Choosing w = 1/2 + i √3/2, we get the solution
x = w + 1/w = (1/2 + i √3/2) + (1/2 - i √3/2) = (1/2)+(1/2) = 1
Also, in this particular example, using complex numbers is optional. By choosing the w= -1 solution to w^3= -1 we successfully avoided dealing with them.