1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Simplifying complex numbers

  1. May 20, 2008 #1
    1. The problem statement, all variables and given/known data

    Simplify [itex](1+i\sqrt{2})^5-(1-i\sqrt{2})^5[/itex]

    2. Relevant equations





    3. The attempt at a solution

    How will I get integer angle out of here?

    [tex]arccos\frac{\sqrt{3}}{3} \approx 54.74^\circ[/tex]

    [tex]arcsin\frac{\sqrt{-6}}{3} \approx -54.74^\circ [/tex]
  2. jcsd
  3. May 20, 2008 #2


    User Avatar
    Homework Helper

  4. May 20, 2008 #3
    Yes, I know de Moivre's theorem, but I don't know how will I get integer at the end...
  5. May 20, 2008 #4
    Why do you need an integer at the end? Is this a part of the question that hasn't been specified? ???
  6. May 20, 2008 #5


    [tex]9\sqrt{3}(0.065-0.997i)-9\sqrt{3}(0.065+0.997i)=9\sqrt{3}(0.065-0.997i-0.065-0.997i)=9\sqrt{3}(-2*0.997i)=-i17.946\sqrt{3} \approx -31.08i[/tex]

    And in my text book results: [itex]-22i\sqrt{2}[/itex], we both get same result, but the question is how they get integer numbers?
  7. May 21, 2008 #6


    User Avatar
    Homework Helper

    I can think of one possible way to get that: Expand the complex numbers out by binomial theorem and then simplify the expression. This would be very tedious, no doubt.
  8. May 21, 2008 #7
    Defennder, I know that I can solve it like that, but it is far more complicated, and the possibility that you may miss some value is very big...
  9. May 21, 2008 #8


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I think it was be best to put the two complex numbers into exponential form for the powers and then convert back to Cartesian to perform the subtraction.
  10. May 22, 2008 #9


    User Avatar
    Science Advisor
    Homework Helper

    Oh come on, guys! :rolleyes:

    :smile: (a + b)^5 - (a - b)^5 = … ? :smile:
  11. May 22, 2008 #10
    By binom formula?
  12. May 22, 2008 #11


    User Avatar
    Science Advisor

    Have you tried it?
  13. May 22, 2008 #12


    User Avatar
    Homework Helper

    Well actually he's asking for a quicker way to get the answer in terms of radicals apart from the binomial theorem or de Moivre's theorem
  14. May 22, 2008 #13
    Tiny Tim is right to point this out. But the explanation for a general rule of a difference of this sort is in line, for learning purposes of course ;)

    If you have an equation of the above kind e.g. (a+b)^n - (a-b)^n , an expansion shows that there will be n+1 terms for (a+b)^n and (a-b)^n. The difference of the two, however, eliminates all but the even terms: for these terms, the coefficients are doubled.
    Let's look at an easier example, (a+b)^3 - (a-b)^3 .
    Using the binomial theorem (by writing the coefficients as they would appear in Pascale's triangle and by ordering terms by increasing exponents of b and decreasing of a) we get

    [a^3 + 3(a^2)(b)+3(a)(b^2) + b^2] - [a^3 - 3(a^2)(b)+3(a)(b^2) -b^3]
    = 6 (a^2)(b)+ 2 b^3

    As you can see, this is just twice the even-numbered terms in the expansion of (a+b)^3.
    The case for (a+b)^5 is analogous.

    This is a succinct way of arriving at the result.
  15. May 22, 2008 #14
    Ahh... I understand now. So I should also use the binom formula, right?
  16. May 22, 2008 #15


    User Avatar
    Science Advisor
    Homework Helper

    :smile: … and don't answer with a question … ! :smile:
  17. May 22, 2008 #16
    [tex]-22i\sqrt{2}[/tex]. :smile: I know that, but I should have do that with De Moivre's formula.
  18. May 22, 2008 #17


    User Avatar
    Science Advisor
    Homework Helper

    erm … no.
    eh? … but this is Physicsissuef's question, not yours! :confused:

    What makes you think he has to use de Moivre?
  19. May 22, 2008 #18
    Since I have same problem in my book. And I solve it correctly, why you say no?
  20. May 23, 2008 #19
    Yes, since logically we need to solve this problem as simple as possible, but no problem.
  21. May 23, 2008 #20


    User Avatar
    Science Advisor
    Homework Helper

    oops! it is -22i√2. Sorry! :redface:

    If you must do it by de Moivre, just put (1 + i√2) = r(cosθ + i sinθ), but leave putting the numbers in until the end.

    Then you want r^5[(cos5θ + i sin5θ) - (cos5θ - i sin5θ)], = 2 i r^5 sin5θ.

    You know r = √3, and tanθ = √2, so it's fairly easy to work out from that what sin5θ is. :smile:

    (But the binomial method is probably a more straightforward way of calculating sin5θ, sin 7θ, etc)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook