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Homework Help: Simple complex number question

  1. Oct 29, 2008 #1
    1. The problem statement, all variables and given/known data
    3. Write down the (x+iy) form for the complex numbers with the following modulus and argument (in radians):
    (a) Modulus 1, argument pi
    (b) Modulus 3, argument -pi/3
    (c)Modulus 7, argument -4

    2. Relevant equations
    Modulus = ((a)^2 + (b)^2)^1/2
    Arg = the angle

    3. The attempt at a solution
    I'm basically just checking to see if i've got the right method.

    (a)If Modulus = 1 and the argument = pi, then that would mean the angle is theta = 0. The only way to achieve this would to be if the imaginary part = 0, so the answer is:
    1+0i or just 1

    (b)This is where I start to get confused. If the argument = -pi/3 then I can use pythagoras to find the adjacent and hypoteneuse.
    In this case:
    cos(-120)*3 = a
    a = -3/2

    sin(-120)*3 = b
    b = -3((3)^1/2)/2

    This works since ((a)^2 + (b)^2)^1/2 does give me 3 in this case

    (c)Modulus 7, Argument is -4, which gives me an angle of (-229.183'). So I can do the same again.

    cos(-229.183)*7 = a
    a = -6.918

    sin(-229.183)*7 = b
    b = -1.067

    This once again seems to work, so my answerwould be something like -6.918 + -1.067i

    I was wanting to check if this is all correct or if i've done the question completely wrong.

    Thanks alot
  2. jcsd
  3. Oct 29, 2008 #2


    Staff: Mentor

    For a) arg(1 + 0i) = 0, not pi, so 1 + 0i is not correct.

    For b) your answer looks correct. An arg of -pi/3 puts the complex number in the 3rd quadrant, so both the Re part and the Im part will have negative values.

    For c) your Re value is close (to get closer, use more decimal place precision in your conversion from radians to degrees) but the Im value has the wrong sign. An arg of -4 puts the complex number in the 2nd quadrant. -pi > -4 > -3pi/2, where '>' signifies "is less negative than"
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