Are these all real?

Algebra Level 2

x = x 2 \sqrt{x}=x^2

Given that x 1 x_1 and x 2 x_2 are the real solutions to the equation above and that x 3 x_3 and x 4 x_4 are the complex ones, calculate x 1 + x 2 + x 3 + x 4 x_1 + x_2 + |x_3| + |x_4|

Notation: z |z| means the magnitude of z z where z C z\in\mathbb{C} .


The answer is 3.

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2 solutions

Ved Pradhan
Jul 4, 2020

x = x 2 \sqrt{x}=x^{2} x = x 4 x=x^{4} x 4 x = 0 x^{4}-x=0 x ( x 3 1 ) = 0 x(x^{3}-1)=0 x = 0 or x 3 1 = 0 x=0\text{ or }x^{3}-1=0

The first part has one solution, as stated. The second part has three solutions, all of which have a magnitude (or absolute value) of 1 1 . It is not necessary to calculate the actual solutions, as long as you can visualize the magnitude. Thus, we have one solution with a magnitude of 0 0 and three solutions with a magnitude of 1 1 . Thus, the sum of the magnitude of the solutions is 1 × 0 + 3 × 1 = 3 1 \times 0 + 3 \times 1 = \boxed{3} .

Aryan Sanghi
Jul 5, 2020

It can be factored as

x = x 4 x = x^4

x ( x 3 1 ) = 0 x(x^3 - 1) = 0

Now, three roots are 0 , 1 , ω , ω 2 0, 1, \omega, \omega^2 where ω = imaginary cube root of unity \omega = \text{ imaginary cube root of unity} .

Now ω = ω 2 = 1 \text{Now } |\omega| = |\omega^2| = 1

Therefore 0 + 1 + ω + ω 2 = 3 \text{Therefore }\color{#3D99F6}{\boxed{0 + 1 + |\omega| + |\omega^2| = 3}}

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