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Mathematics 15 Online
OpenStudy (anonymous):

(1)^1/3=?

OpenStudy (anonymous):

I know simple, but can't think right now

OpenStudy (anonymous):

im pretty sure it would be 1!

OpenStudy (anonymous):

I was thinking 1/3 but not quite sure

OpenStudy (anonymous):

nope lol can you try 1 then let me know because i took (1)^1/3 and i got 1 after i typed it in my calculator

OpenStudy (anonymous):

On my last try, My calculator computes .333333 aka 1/3. try parenthesis around the one and then make sure you are placing 1/3 as a power/exponent outside the parenthesis

OpenStudy (anonymous):

ok hold on lol

OpenStudy (anonymous):

ok nevermind i got 1/3. this time lol

OpenStudy (anonymous):

ok just making sure, how r u at trig?

OpenStudy (anonymous):

yupp and ummm... it depends what the problem is lol

OpenStudy (anonymous):

depends on what class you are taking if you are using real numbers then \[1^{\frac{1}{3}}=\sqrt[3]{1}=1\]

OpenStudy (anonymous):

if you are living in the world of complex numbers, then there are three "cube roots" of one

OpenStudy (anonymous):

complex

OpenStudy (anonymous):

i'll display the whole problem

OpenStudy (anonymous):

on of them is 1 the other two are \(-\frac{1}{2}+\frac{\sqrt{3}}{2}i\) and its conjugate \(-\frac{1}{2}-\frac{\sqrt{3}}{2}i\)

OpenStudy (anonymous):

Solve the equation. (List your answers counterclockwise about the origin starting at the positive real axis. Express θ in radians.) z3 − 1 = 0

OpenStudy (anonymous):

there are a couple ways to find the other two. from algebra you can solve \[x^3=1\] \[x^3-1=0\] \[(x-1)(x^2+x+1)=0\] and use the quadratic formula for \(x^2+x+1=0\)

OpenStudy (anonymous):

or you can use trig

OpenStudy (anonymous):

or you can just look with your eyes at the unit circle divided in two three parts with one of the parts at 1 |dw:1355147845893:dw|

OpenStudy (anonymous):

Its trig, using unit circle pi/3, pi, 5pi/3

OpenStudy (anonymous):

no it would not be those it would be the ones on the picture above the angles are \(\theta=0,\theta=\frac{2\pi}{3},\theta=\frac{4\pi}{3}\)

OpenStudy (anonymous):

you can check it is right using the quadratic formula, but we can work through the trig slowly if you like

OpenStudy (anonymous):

the radius would b? the equation will be in this format, r(cos \[r(\cos \theta + isin \theta) ____ z ^{3}-1=0 _____ z ^{3}=1\]

OpenStudy (anonymous):

ok i got it

OpenStudy (anonymous):

the radius is 1

OpenStudy (anonymous):

because you are trying to solve \(z^3=1\)

OpenStudy (anonymous):

\[\sqrt[3]{z}= 1^{\frac{ 1 }{ 3 }} _____ 1/3(\cos \pi/3+isin \pi/3)\]

OpenStudy (anonymous):

lets go slow, i sense there is some confusion about what you are trying to solve and how you write a number in trig form

OpenStudy (anonymous):

the \(\frac{1}{3}\) is in the exponent, it is not a coefficient

OpenStudy (anonymous):

your job is to solve \(z^3=1\) in the complex plane, i.e. your job is to find the three cube roots of one we can write 1 as a complex number in the form of \(1=\cos(0)+i\sin(0)\) which is a rather silly thing to do but it has it uses we know this because \(\cos(0)=1\) and \(\sin(0)=0\) so this just says \(1=1+0i\)

OpenStudy (anonymous):

to find the cube root, divide the angle by 3 and take the cube root of \(r\) which is in this case its 1 but zero divided by 3 is just zero, and the cube root of one is just one so again you get \[\cos(0)+i\sin(0)=1\] which you already knew, since it is clear that \(1^3=1\) the real job it to find the other two cubed roots of one

OpenStudy (anonymous):

if you go around the unit circle again, you go from \(0\) to \(2\pi\) and so you can write \[1=\cos(2\pi)+i\sin(2\pi)\] now again divide the angle by 3 for the next cubed root of 1 and get \[\cos(\frac{2\pi}{3})+i\sin(\frac{2\pi}{3})\]

OpenStudy (anonymous):

when you evaluate the trig functions you get \[\cos(\frac{2\pi}{3})+i\sin(\frac{2\pi}{3})=-\frac{1}{2}+\frac{\sqrt{3}}{2}i\]

OpenStudy (anonymous):

let me know when i have lost you

OpenStudy (anonymous):

got u its becoming clearer

OpenStudy (anonymous):

Thank u so much

OpenStudy (anonymous):

then if you go around the unit circle one more time, you get \[1=\cos(4\pi)+i\sin(4\pi)\] divide by 3 get \[\cos(\frac{4\pi}{3})+i\sin(\frac{4\pi}{3})\]

OpenStudy (anonymous):

it is really much easier to visualize dividing the unit circle in to three parts see the picture above

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