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Question Number 136167 by ZiYangLee last updated on 19/Mar/21
Given that one of the value for the 5^(th)   root of a complex number is −1+i.  Find the another four values.
$$\mathrm{Given}\:\mathrm{that}\:\mathrm{one}\:\mathrm{of}\:\mathrm{the}\:\mathrm{value}\:\mathrm{for}\:\mathrm{the}\:\mathrm{5}^{\mathrm{th}} \\ $$$$\mathrm{root}\:\mathrm{of}\:\mathrm{a}\:\mathrm{complex}\:\mathrm{number}\:\mathrm{is}\:−\mathrm{1}+{i}. \\ $$$$\mathrm{Find}\:\mathrm{the}\:\mathrm{another}\:\mathrm{four}\:\mathrm{values}. \\ $$
Answered by mr W last updated on 19/Mar/21
one 5^(th)  root is −1+i=(√2)(cos ((3π)/4)+i sin ((3π)/4))  ((3π)/4)+((2π)/5)=((23π)/(20))  ((3π)/4)+((2π)/5)+((2π)/5)=((31π)/(20))  ((3π)/4)+((2π)/5)+((2π)/5)+((2π)/5)=((39π)/(20))  ((3π)/4)−((2π)/5)=((7π)/(20))  the other 5^(th)  roots are:  (√2)[cos (((7π)/(20)))+i sin (((7π)/(20)))]  (√2)[cos (((23π)/(20)))+i sin (((23π)/(20)))]  (√2)[cos (((31π)/(20)))+i sin (((31π)/(20)))]  (√2)[cos (((39π)/(20)))+i sin (((39π)/(20)))]
$${one}\:\mathrm{5}^{{th}} \:{root}\:{is}\:−\mathrm{1}+{i}=\sqrt{\mathrm{2}}\left(\mathrm{cos}\:\frac{\mathrm{3}\pi}{\mathrm{4}}+{i}\:\mathrm{sin}\:\frac{\mathrm{3}\pi}{\mathrm{4}}\right) \\ $$$$\frac{\mathrm{3}\pi}{\mathrm{4}}+\frac{\mathrm{2}\pi}{\mathrm{5}}=\frac{\mathrm{23}\pi}{\mathrm{20}} \\ $$$$\frac{\mathrm{3}\pi}{\mathrm{4}}+\frac{\mathrm{2}\pi}{\mathrm{5}}+\frac{\mathrm{2}\pi}{\mathrm{5}}=\frac{\mathrm{31}\pi}{\mathrm{20}} \\ $$$$\frac{\mathrm{3}\pi}{\mathrm{4}}+\frac{\mathrm{2}\pi}{\mathrm{5}}+\frac{\mathrm{2}\pi}{\mathrm{5}}+\frac{\mathrm{2}\pi}{\mathrm{5}}=\frac{\mathrm{39}\pi}{\mathrm{20}} \\ $$$$\frac{\mathrm{3}\pi}{\mathrm{4}}−\frac{\mathrm{2}\pi}{\mathrm{5}}=\frac{\mathrm{7}\pi}{\mathrm{20}} \\ $$$${the}\:{other}\:\mathrm{5}^{{th}} \:{roots}\:{are}: \\ $$$$\sqrt{\mathrm{2}}\left[\mathrm{cos}\:\left(\frac{\mathrm{7}\pi}{\mathrm{20}}\right)+{i}\:\mathrm{sin}\:\left(\frac{\mathrm{7}\pi}{\mathrm{20}}\right)\right] \\ $$$$\sqrt{\mathrm{2}}\left[\mathrm{cos}\:\left(\frac{\mathrm{23}\pi}{\mathrm{20}}\right)+{i}\:\mathrm{sin}\:\left(\frac{\mathrm{23}\pi}{\mathrm{20}}\right)\right] \\ $$$$\sqrt{\mathrm{2}}\left[\mathrm{cos}\:\left(\frac{\mathrm{31}\pi}{\mathrm{20}}\right)+{i}\:\mathrm{sin}\:\left(\frac{\mathrm{31}\pi}{\mathrm{20}}\right)\right] \\ $$$$\sqrt{\mathrm{2}}\left[\mathrm{cos}\:\left(\frac{\mathrm{39}\pi}{\mathrm{20}}\right)+{i}\:\mathrm{sin}\:\left(\frac{\mathrm{39}\pi}{\mathrm{20}}\right)\right] \\ $$
Commented by mr W last updated on 19/Mar/21
Commented by mr W last updated on 20/Mar/21
■ a known n^(th)  root of z  ■ the other n^(th)  roots of z  if one of the n^(th)  roots of a number is  re^(αi) , then the other n^(th)  roots of it are  re^((α+((2kπ)/n))i)  with k=1,2,...,n−1
$$\blacksquare\:{a}\:{known}\:{n}^{{th}} \:{root}\:{of}\:{z} \\ $$$$\blacksquare\:{the}\:{other}\:{n}^{{th}} \:{roots}\:{of}\:{z} \\ $$$${if}\:{one}\:{of}\:{the}\:{n}^{{th}} \:{roots}\:{of}\:{a}\:{number}\:{is} \\ $$$${re}^{\alpha{i}} ,\:{then}\:{the}\:{other}\:{n}^{{th}} \:{roots}\:{of}\:{it}\:{are} \\ $$$${re}^{\left(\alpha+\frac{\mathrm{2}{k}\pi}{{n}}\right){i}} \:{with}\:{k}=\mathrm{1},\mathrm{2},…,{n}−\mathrm{1} \\ $$
Commented by ZiYangLee last updated on 20/Mar/21
Thanks Sir!
$$\mathrm{Thanks}\:\mathrm{Sir}! \\ $$

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