Question #117208
2. (a) Find the real root of the equation z3 + z + 10 = 0 given that one root is 1 − 2i.
Show that z = i is a root of the equation z4 + z3 + z − 1 = 0. Find the three
other roots.
1
Expert's answer
2020-05-26T18:13:19-0400

z=1+2iz=1+2i is also a root of equation. We obtain this by conjugating the equation. Dividing the polynomial z3+z+10z^3+z+10 by (z12i)(z1+2i)=z22z+5(z-1-2i)(z-1+2i)=z^2-2z+5

we receive z+2z+2 . Thus, z=2z=-2 is the real root.


Now we consider equation z4+z3+z1=0z^4+z^3+z-1=0 . We will check that z1=iz_1=i is a root of equation. Namely, we have: z14+z13+z11=1i+i1=0z_1^4+z_1^3+z_1-1=1-i+i-1=0

It is clear that z2=iz_2=-i is also a root of equation.

This can be obtained from the fact that z2=zˉ1z_2=\bar{z}_1

We shall divide the equation by (zi)(z+i)=z2+1(z-i)(z+i)=z^2+1 .

Then one receives z21+z.z^2-1+z. It remains to solve z2+z1=0z^2+z-1=0 .

The latter quadratic equation has solutions (see e.g. https://en.wikipedia.org/wiki/Quadratic_equation for detalis)

z3=1+52,z4=152z_3 = \frac{-1+\sqrt{5}}2, \,\,z_4 = \frac{-1-\sqrt{5}}2 .


Thus, we have the roots z1=i,z2=i,z3=1+52,z4=152,z_1=i,\quad z_2=-i,\quad z_3 = \frac{-1+\sqrt{5}}2,\quad z_4 = \frac{-1-\sqrt{5}}2,



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