x ( x-7)² = ( x-15)²
x³-15 x²+79x -225 =0 daca z₁= 3+4i atunci z₂= 3 -4i
si ecuatia se divide cu ( x-3-4i)·( x-3+4i) I ecuatia
(x² -6x+25) I ecuatia
x³ -15 x² +79x -225 I x² -6x+25
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-x³+6x²-25x x-9
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-9x²+54 x -225
+9x² -54x +225
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/ / /
a treia radacina x-9 =0 x=9