De Moivre's Theorem (-12-5i)^-3

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In summary, to solve (-12-5i)^-3 using De Moivres Theorem, first rewrite the complex number in polar form as z = 13cis(theta), with theta = tan^{-1}(-5/-12) - \pi. Then use De Moivres Theorem to find the solution as 1/2197cis(-24.723).
  • #1
Wardlaw
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Homework Statement



Using De Moivres Theorem, solve (-12-5i)^-3

Homework Equations





The Attempt at a Solution



The solution i get for this problem is different from the one given in the exercise text. This is 1/2197cis(8.241)

Note: cis is equivalent to cos([tex]\Theta[/tex])+isin([tex]\Theta[/tex])
 
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  • #2
well you haven't actually written an equation down so there's nothing to solve.
i'm assuming you meant to rewrite it in polar form and then use de moivre

write -12-5i in polar form, that is z=r cis(theta)
where r will be 13 if my mental pythagoras is correct and theta is arctan(y/x)

then use de moivre (-12-5i)^(-3)=z^(-3)=r^(-3) cis(-3 theta)
 
  • #3
latentcorpse said:
well you haven't actually written an equation down so there's nothing to solve.
i'm assuming you meant to rewrite it in polar form and then use de moivre

write -12-5i in polar form, that is z=r cis(theta)
where r will be 13 if my mental pythagoras is correct and theta is arctan(y/x)

then use de moivre (-12-5i)^(-3)=z^(-3)=r^(-3) cis(-3 theta)

Remember that in the Argand diagram,-12-5i lies in the 3rd quadrant. Thus

[tex]\theta = tan^{-1}(y/x) - \pi[/tex]
 

FAQ: De Moivre's Theorem (-12-5i)^-3

What is De Moivre's Theorem?

De Moivre's Theorem is a mathematical theorem that relates to complex numbers. It states that for any complex number z and integer n, (cos θ + i sin θ)n = cos nθ + i sin nθ, where θ is the argument of the complex number.

What is the formula for De Moivre's Theorem?

The formula for De Moivre's Theorem is (cos θ + i sin θ)n = cos nθ + i sin nθ, where θ is the argument of the complex number and n is an integer.

How is De Moivre's Theorem used?

De Moivre's Theorem is used to simplify calculations involving complex numbers by converting them from polar form to rectangular form. It is also used in solving equations involving complex numbers.

Can De Moivre's Theorem be applied to negative exponents?

Yes, De Moivre's Theorem can be applied to negative exponents. For example, (cos θ + i sin θ)-n = cos -nθ + i sin -nθ = cos nθ - i sin nθ.

How do you use De Moivre's Theorem to find the nth root of a complex number?

To find the nth root of a complex number using De Moivre's Theorem, you would first convert the complex number to polar form, then apply the formula (r(cos θ + i sin θ))1/n = r1/n(cos (θ/n) + i sin (θ/n)). This will give you the nth roots of the complex number in polar form, which can then be converted back to rectangular form if needed.

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