Determine all \(\displaystyle{z}\in{\mathbb{{{C}}}}\) such that \(\displaystyle{z}^{{8}}+{3}{i}{z}^{{4}}+{4}={0}\)

Dominique Pace

Dominique Pace

Answered question

2022-03-27

Determine all zC such that z8+3iz4+4=0

Answer & Explanation

Alannah Campos

Alannah Campos

Beginner2022-03-28Added 10 answers

This is a nice question. Notice that if we relabel z4 as w then we have:
z8+3iz4+4w2+3iw+4
We can use the quadratic formula to solve w2+3iw+4=0, where a=1, b=3i and c=4
w=b±b24ac2a
=3i±9162
=3i±5i2
=4i or i
Since w2+3iw+4=0w[4i,i] and w=z4, you're left needing to solve z4=i and z4=4i. First, consider the case z4=i. We know that i=ei(π2+2πn) hence:
z=i14
=ei(π8+πn2)
=e5i8, e9i8, e13i8, e9i4
Now do the same to solve z4=4i

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