Number of roots of a complex equation

Algebra Level 3

How many complex number z z that satisfies the equation below?

z ( z 4 i ) + 2 i = ( 5 i ) z \large |z|(z - 4 - i) + 2i = (5 - i)z

1 3 2 4 0

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1 solution

Russell Ngo
Aug 16, 2018

z ( z 4 i ) + 2 i = ( 5 i ) z z ( z 5 + i ) = 4 z + ( z 2 ) i |z|(z - 4 - i) + 2i = (5-i)z \Leftrightarrow z(|z| - 5 + i) = 4|z| + (|z| - 2)i

Take the modulus of both sides, we get:

z ( z 5 ) 2 + 1 = ( 4 z ) 2 + ( z 2 ) 2 |z|\sqrt{(|z|-5)^2+1} = \sqrt{(4|z|)^2+(|z| - 2)^2}

Make the substitution t = z t=|z| , the above equation becomes:

t ( t 5 ) 2 + 1 = ( 4 t ) 2 = ( t 2 ) 2 ( t 1 ) ( t 3 9 t 2 + 4 ) = 0 t\sqrt{(t-5)^2+1} = \sqrt{(4t)^2 = (t-2)^2} \Leftrightarrow (t - 1)(t^3 - 9t^2 + 4) = 0

That cubic equation has 3 distinct roots t 0 t\geq0 , so there are 3 complex numbers z z that satisfies the equation

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