SOLUTION: z^2=5-12i Where z=a+bi How to do it, help ls

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Question 1046823: z^2=5-12i
Where z=a+bi
How to do it, help ls

Found 3 solutions by Alan3354, ankor@dixie-net.com, ikleyn:
Answer by Alan3354(69443) About Me  (Show Source):
Answer by ankor@dixie-net.com(22740) About Me  (Show Source):
You can put this solution on YOUR website!
z^2=5-12i
Where z=a+bi
:
Replace z with (a+bi)
(a+bi)^2 = 5 - 12i
:
FOIL (a+bi)(a+bi)
a^2 + 2abi + b^2i^2 = 5 -12i
:
i^2 = -1 therefore
a^2 + 2abi - b^2 = 5 - 12i
a^2 + 2abi = b^2 + 5 - 12i
a^2 + 2abi + 12i = b^2 + 5
:
I don't know what else you can do with it.

Answer by ikleyn(52887) About Me  (Show Source):
You can put this solution on YOUR website!
.
z^2=5-12i
Where z=a+bi
How to do it, help ls
~~~~~~~~~~~~~~~~~~~~

z = a + bi   --->  z%5E2 = %28a%2Bbi%29%5E2 = a%5E2+%2B+2abi+%2B+%28bi%29%5E2 = a%5E2+-+b%5E2+%2B+2abi    (since %28bi%29%5E2 = b%5E2%2Ai%5E2 = b%5E2%2A%28-1%29 = -b%5E2   //  <--- since i%5E2 = -1)

Now you have, from the condition, an equality of these two complex numbers:

a%5E2+-+b%5E2+%2B+2abi = 5 - 12i.

It means that the real parts are equal and the imaginary parts are equal:

a%5E2+-+b%5E2 = 5,     (1)
2ab = -12.       (2)


It is the system of two equations for the two unknowns "a" and "b".
To solve it, express b = -12%2F%282a%29 = -6%2Fa from (2) and substitute it into (1). You will get

a%5E2+-+%28-6%2Fa%29%5E2 = 5,    or

a%5E2 - 36%2Fa%5E2 = 5.

Multiply both sides by a^2. You will get

a%5E4+-+5a%5E2+-+36 = 0.

Factor the left side

%28a%5E2-9%29%2A%28a%5E2%2B4%29 = 0.


The equation deploys in two independent equations


1.  a%5E2-9 = 0  --->  a%5E2 = 9  --->  a = +/- 3  --->  b = -6/a = -/+ 2.


2.  a%5E2%2B4 = 0  --->  a%5E2 = -4  --->  No real solutions.


Answer. The solutions of the original equation z%5E2 = 5 - 12i are these two complex numbers: z%5B1%5D = 3 - 2i  or/and  z%5B2%5D = -3 + 2i.

       In other words, the square root of the complex number 5 - 12i in the complex domain are these two complex numbers: 3 - 2i  and  -3 + 2i.

There is a bunch of my lessons on complex numbers
    - Complex numbers and arithmetical operations on them
    - Complex plane
    - Addition and subtraction of complex numbers in complex plane
    - Multiplication and division of complex numbers in complex plane
    - Raising a complex number to an integer power
    - How to take a root of a complex number
    - Solution of the quadratic equation with real coefficients on complex domain
    - How to take a square root of a complex number
    - Solution of the quadratic equation with complex coefficients on complex domain
in this site.