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Homework Help: Complex numbers. write equation on form "a+bi"

  1. Sep 1, 2016 #1
    1. The problem statement, all variables and given/known data
    Write this complex number in the form "a+bi"
    a) cos(-pi/3) + i*sin(-pi/3)
    b) 2√2(cos(-5pi/6)+i*sin(-5pi/6))
    2. Relevant equations
    my only problem is that im getting + instead of - on the cosinus side.(real number)

    3. The attempt at a solution
    a) pi/3 in the unit circle is 1/2 for cosinus and √3)/2 for sinus, and since both have minus infront it should be
    my answer = -1/2-√3/2*i
    the solution: 1/2-√3/2*i

    b) exactly the same on b. -5pi/6 in the unit circle is -√3/2 for cos, and 1/2 for sin.
    2√2*-(-√3/2) + i*2√2*-1/2
    removing two's over and under. √2*√3-√2*i
    my answer : = √6 -√2*i
    the solution is -√6 -√2*i

    sorry to bother,
    thanks, T
  2. jcsd
  3. Sep 1, 2016 #2
    What is ##cos(-\theta)##?
    Edit: And ##sin(-\theta)##
    Last edited: Sep 1, 2016
  4. Sep 1, 2016 #3
    lol, thanks for the help :D
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