Roots of -8: Solving with Exponential Form

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In summary, the conversation discusses finding the 6 roots of -8 through the use of complex numbers and exponential form. The solution given for the question has a minus sign in the exponent, but it is determined that both expressions are equivalent. The conversation also mentions the use of the principle argument and how it affects the answer.
  • #1
31415
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Homework Statement



I'm supposed to find the 6 roots of -8 + 0i

we are told the usual method of for these problems is to put the complex number into it's exponential form like so

z = |z|exp( i(θ+2πk)) where k: [0 to n-1]

then put it to the relevant power 'n'

z1/n = |z|1/nexp(( i(θ+2πk)/n)

so I proceed by taking θ to be pi...

z1/6 = 81/6exp(( i(π+2πk)/6)

thinking the question is easy...

then in the solution given for the question the answer is 81/6exp(( i(-π+2πk)/6)

I don't understand where this minus has come from - is the solution wrong or am I missing something?

(picture of question and solution attached)
 

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  • #2
Both expressions are the same 6 numbers, it's just that the value of k in one set is shifted by 1 with respect to the other set.
 
  • #3
31415 said:

Homework Statement



I'm supposed to find the 6 roots of -8 + 0i

we are told the usual method of for these problems is to put the complex number into it's exponential form like so

z = |z|exp( i(θ+2πk)) where k: [0 to n-1]

then put it to the relevant power 'n'

z1/n = |z|1/nexp(( i(θ+2πk)/n)

so I proceed by taking θ to be pi...

z1/6 = 81/6exp(( i(π+2πk)/6)

thinking the question is easy...

then in the solution given for the question the answer is 81/6exp(( i(-π+2πk)/6)

I don't understand where this minus has come from - is the solution wrong or am I missing something?

You could always use
$$
w_k = r^{1/n}\left[\cos\frac{\theta + 2\pi k}{n}+i\sin\frac{\theta + 2\pi k}{n}\right]
$$
where ##n## is the number of roots in your case 6, ##\theta## is the angle, and ##k = 0,1,..,n-1##

Then ##r = 8## and ##\theta = \pi## where the principle argument is define as ##\theta\in (-\pi,\pi]##
So
$$
w_k = 8^{1/6}\left[\cos\frac{\pi + 2\pi k}{6}+i\sin\frac{\pi + 2\pi k}{6}\right] = 8^{1/6}\exp\left[i\frac{\pi + 2\pi k}{6}\right]
$$

So how could ##\pi## be negative? Simply define the the principle argument to be ##\theta\in [-\pi,\pi)##.

And sometimes it will be defined be excluding ##\pi## all together like ##\theta\in (-\pi,\pi)##
 
  • #4
what minus sign? I see the -8+0i which you've handled by using pi for theta.

your answer looks right as it is.
 
  • #5
ah i see when I take the principle arguments of each root then the answers agree - thanks all. Still don't understand why he decided to do it like that but I guess it doesn't matter :)
 
  • #6
attachment.php?attachmentid=47370&d=1337277366.jpg


You're supposed to find the 6 sixth roots of -8 .
 
  • #7
SammyS said:
You're supposed to find the 6 sixth roots of -8 .

yeah I know that - ... e^i*pi = -1

8 e^i*pi = -8

e^2*pi*k =1 for k: 0,1,2,3...
 
  • #8
and when you plot them in the complex plane it will look like a six evenly spaced slices of a pizza pi with a centerline about the imaginary axis.
 
  • #9
And of course you can write ##\sqrt{2}## in place of ##8^{1/6}## in your answer.
 

FAQ: Roots of -8: Solving with Exponential Form

What is the exponential form for the square root of -8?

The exponential form for the square root of -8 is √(-8) = (-8)^(1/2).

How do you simplify the expression (-8)^(1/2)?

To simplify (-8)^(1/2), you can rewrite it as (√(-8))^2 and then use the property that (√a)^2 = a to get √(-8) = √(-1*2*2*2) = 2i√2.

Can you solve for the roots of -8 using only exponential form?

Yes, the roots of -8 can be solved using only exponential form. The two roots are √(-8) = 2i√2 and -√(-8) = -2i√2.

How do you graph the roots of -8 in the complex plane?

To graph the roots of -8 in the complex plane, you can plot the points (0, 2√2) and (0, -2√2) on the imaginary axis. These points represent the two roots, 2i√2 and -2i√2, respectively.

What is the relationship between the roots of -8 and the imaginary unit i?

The roots of -8, 2i√2 and -2i√2, are both complex numbers that involve the imaginary unit i. This is because the square root of -8 cannot be simplified to a real number, so it must be expressed in terms of i.

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