.
= = = . (1)
The factor also can be factored over real numbers
= - = - = .
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| Far not everyone knows about this tricky decomposition; but those who are trained in Math, know it. |
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Therefore, decomposition (1) can be continue farther
= = = .
Three remaining quadratic polynomials CAN NOT be factored further over real numbers.
Therefore, k = 5 is the largest value of " k " under the problem's question. ANSWER
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Solved.
The answer and the statement " k = 4 " by @greenestamps is not correct.