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brcole
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Homework Statement
Determine all z ∈ C such that z^3-i = 0.
Homework Equations
z^3-i=0
The Attempt at a Solution
z = -(i^3) = -((0+i)^3)
Dick said:The equation you want to solve is z^3=i, not z=(-i)^3. Write i in polar form. i=e^(pi*i/2). Does that sound familiar?
Mentallic said:Do you know how to factorize the sum/difference of 2 cubes?
[tex]a^3\pm b^3=(a\pm b)(a^2\mp ab+b^2)[/tex]
Once you've applied this, you will have a root and a quadratic (the 2nd factor). Just apply the quadratic formula and you will then have your 3 complex roots.
gabbagabbahey said:How exactly is this useful here?...b would be [tex]i^{1/3}[/tex], but that is exactly what he is trying to find.
Mentallic said:This reminds of how some of my classmates would say "but x^3-1 can't be factorised because we need the difference of 2 cubes"
So we have [tex]z^3-i=0[/tex]
[tex]i=\sqrt{-1}[/tex]
[tex]i^2=-1[/tex]
[tex]i^3=-\sqrt{-1}=-i[/tex]
Thus, this can be transformed into the difference of 2 cubes such that:
[tex]z^3+i^3=0[/tex]
Dick said:This is all true. You can do it that way. But you going to have a hard time applying that to the more general problem of solving z^n=C. Unless i) you can guess an nth root for C (I think that was gabbagabbahey's point) and ii) you can solve the other n-1 degree equation. I think it's better to take the general approach.
Mentallic said:Taking the general approach is all fine and dandy, but using it constantly for all questions that appear in that form... well... it can cloud your mind and you wouldn't even look twice at the question to see if there is an easier and less prone-to-failure approach.
Once again, this reminds me of my class on a similar matter. We were asked to solve for x: [tex](x+4)^2=3[/tex] and wouldn't you know it, half the class took the 'general approach' by expanding out, simplifying, and using the quadratic equation. There were many more algebraic errors coming from the half that expanded than from the half that instantly realized the square roots could be taken from both ends.
All in all, for this question, I would recommend factorizing.
Dick said:Well, I don't. If the 'factoring' led to to a trivial solution, I would say, sure. But it doesn't. Solving the quadratic part involves finding the square root of a complex number, which puts you right back into much the same sort of a boat you were into begin with. I don't think it simplifies it. It's much easier to find the arguments by dividing angles by three.
Dick said:You're right. My mistake. I had the i in the wrong place in the quadratic. It is a reasonable sneaky way to get a quick answer. Sorry. If the OP hasn't been back before now, they are probably not coming back.
A complex equation is an equation that involves complex numbers, which are numbers that have both a real and imaginary component. It can be written in the form a + bi, where a is the real part and bi is the imaginary part (with i being the imaginary unit).
To solve a complex equation, you can use algebraic techniques such as combining like terms, distributing, and solving for the variable. You can also use graphical methods, like plotting the complex numbers on a complex plane, or using the quadratic formula.
Some common techniques used to solve complex equations include substitution, elimination, and factoring. You can also use trigonometric identities, logarithms, and the rules of exponents to simplify the equation.
Yes, complex equations can have multiple solutions. In fact, for polynomial equations of degree n, there can be up to n complex solutions. These solutions may include both real and imaginary numbers.
Yes, there are some special cases to consider when solving complex equations. For example, if the equation contains the square root of a negative number, it may have imaginary solutions. Also, if the equation has a variable in the denominator, you need to make sure to avoid any values that would result in a division by zero.