Question #141162
The cubic equation z^3+Az^2+Bz+26=0, A, BeR(A, B are elements in the set R) one root of the above cubic equation is 1+i. Find the real root (ii) Determine the values of A and B.
1
Expert's answer
2020-11-09T20:19:37-0500

First of all, as coefficients A and B are real, we know that if zCz \in \mathbb{C} is a root of an equation, its' complex conjugate zˉ\bar{z} is also a root of this equation :

zˉ3+Azˉ2+Bzˉ+26=z3ˉ+(Az2)ˉ+(Bz)ˉ+26=0ˉ=0\bar{z}^3+A\bar{z}^2+B\bar{z}+26=\bar{z^3}+\bar{(Az^2)}+\bar{(Bz)}+26=\bar{0}=0

Therefore, (1+i)ˉ=1i\bar{(1+i)} = 1-i is also a root of the equation.

Now, to find the third root we can apply, for example, Vieta's formulas :

z1×z2×z3=(1)326z_1\times z_2 \times z_3 = (-1)^3 26

(1i)×(1+i)×z3=26(1-i)\times(1+i)\times z_3 = -26

z3=13z_3 = -13

To find the coefficients we can either replace zz in the equation by the roots we've found (which is more direct method), either reapply the Vieta's formulas :

A=z1z2z3=11A = -z_1-z_2-z_3=11

B=z1z2+z2z3+z1z3=24B = z_1z_2+z_2z_3+z_1z_3 = -24

We can even replace zz in our equation by its' 3 different values and verify that these calculations are correct.


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