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## Homework Statement

Please see the method of solving the equation x^3 -4x^2 + 4x +2 = 0 from the attached.

In the red brackets,can anyone explain why the negative root is not involved in the calculation?

by the quadratic formula,

z = [y±√(y^2 - 4k)]/2

so why only y+√(y^2 - 4k) is considered?

and in the second bracket,again,why the another root of u ,(-1/27)(-35 - 3√129),is not used?

Both roots are negative.

Thx.