SOLUTION: Find the 3 cube roots of i in polar form.

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Question 1180531: Find the 3 cube roots of i in polar form.
Found 2 solutions by ikleyn, Edwin McCravy:
Answer by ikleyn(52807) About Me  (Show Source):
You can put this solution on YOUR website!
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Find the 3 cube roots of i in polar form.
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    i = (1, pi%2F2) = (1,90°)  in polar form  (the modulus is 1;  the argument is  pi%2F2,  or  90°).



Use deMoivre formula.  The three cubic roots of " i " are



a)  (1, pi%2F6) = (1,30°);



b)  (1, pi%2F6+%2B+%282pi%2F3%29) = (1, pi%2F6+%2B+4pi%2F6) = (1, 5pi%2F6) = (1,150°);



c)  (1, pi%2F6+%2B+%284pi%2F3%29) = (1, pi%2F6+%2B+8pi%2F6) = (1, 9pi%2F6) = (1,270°).


Solved and explained.



Answer by Edwin McCravy(20060) About Me  (Show Source):
You can put this solution on YOUR website!
While correct, neither the notation (r,θ) nor rei*θ is used when teaching complex numbers 
in basic trigonometry courses.  That's because such notation eliminates the trigonometric functions.

Find the 3 cube roots of i in polar form.
i = 0 + 1i

Graph the vector whose magnitude (modulus) is r=1, whose tail is at (0,0),
and whose tip is at (0,1), and whose argument (angle) is θ=90o. 



i%22%22=%22%220%2B1%2Ai%22%22=%22%22r%28cos%28theta%29%5E%22%22%2Bi%2Asin%28theta%29%291%28cos%2890%5Eo%29%5E%22%22%2Bi%2Asin%2890%5Eo%29%29

Since the cube root is the 1/3 power:

matrix%282%2C1%2C%22%22%2Ci%5E%281%2F3%29%29matrix%282%2C1%2C%22%22%2C%22%22=%22%22%29

We raise everything to the 1/3 power.  In doing so we will use deMoivre's
theorem, where we raise the magnitude (modulus 1) to the 1/3 power (i.e.,
take its cube root 1), and multiply its argument (angle) by 1/3.



Now, since there are 3 cube roots, we take three consecutive integers for n.

Let n=0

1%28cos%2830%5Eo%2B120%5Eo%2A0%29%5E%22%22%2Bi%2Asin%2830%5Eo%2B120%5Eo%2A0%29%29%22%22=%22%221%28cos%2830%5Eo%29%2Bi%2Asin%2830%5Eo%29%5E%22%22%29

Let n=1

1%28cos%2830%5Eo%2B120%5Eo%2A1%29%5E%22%22%2Bi%2Asin%2830%5Eo%2B120%5Eo%2A1%29%29%22%22=%22%221%28cos%28150%5Eo%29%2Bi%2Asin%28150%5Eo%29%5E%22%22%29

Let n=2

1%28cos%2830%5Eo%2B120%5Eo%2A2%29%5E%22%22%2Bi%2Asin%2830%5Eo%2B120%5Eo%2A2%29%29%22%22=%22%221%28cos%2830%5Eo%2B240%5Eo%29%2Bi%2Asin%2830%5Eo%2B240%5Eo%29%5E%22%22%29%22%22=%22%221%28cos%28270%5Eo%29%2Bi%2Asin%28270%5Eo%29%5E%22%22%29.

Edwin