Complex Numbers#0

Algebra Level 2

Find the number of complex numbers z, such that the sum of z and the reciprocal of its conjugate equals the absolute value of the complex number z.


The answer is 0.

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1 solution

Stephen Mellor
Oct 19, 2017

We can rephrase the problem in numerical form to be:

z + z + 1 z \frac{1}{z*} = z = |z|

By letting z = x + i y z = x + iy and using Pythagoras to evaluate the absolute value:

x + i y + x + iy + 1 x i y \frac{1}{x - iy} = x 2 + y 2 = \sqrt{x^2 + y^2}

Removing the fractions and simplifying:

x 2 + y 2 + 1 = x 2 + y 2 x^2 + y^2 + 1 = \sqrt{x^2 + y^2}

Let: a = x 2 a = x^2 and b = y 2 b = y^2 ,

( a + b + 1 ) 2 = a + b (a + b + 1)^2 = a + b

a 2 + 2 a b + b 2 + 2 a + 2 b + 1 = a + b a^2 + 2ab + b^2 + 2a + 2b + 1 = a + b

a 2 + 2 a b + b 2 = a b 1 a^2 + 2ab + b^2 = - a - b - 1

( a + b ) 2 = a b 1 (a + b)^2 = - a - b - 1

As we have defined z = x + i y z = x + iy , x x and y y are real numbers and since a a and b b are the squares of real numbers, a a and b b are positive real numbers. Therefore, on this last line, the left hand side must be positive and the right hand side must be negative. Hence, there are 0 \boxed{0} values for z z .

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