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Homework Help: Simplify conjugate expressions

  1. Nov 29, 2004 #1
    1) (-7+20)(-10-3i)

    2) (-2-5i)/(3+4i)

    I dont think I did these two questions right does anyone know how?
  2. jcsd
  3. Nov 29, 2004 #2


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    Multiply with 1, written in terms of the conjugate expressions of the denominators.
  4. Nov 29, 2004 #3
    I simplified (-7+2i)(-10-3i) and got 76+i is this correct?
  5. Nov 29, 2004 #4
    yes, that is correct.
  6. Nov 29, 2004 #5
    1) (-7+20)(-10-3i)

    Step 1.) FOIL the Problem
    Step 2.) Simplify from there (Do addition Subtraction etc.)

    2) (-2-5i)/(3+4i)

    I forgot how to do division with imaginary numbers :eek: I'll try to look it back up and give you some help
  7. Nov 29, 2004 #6


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    All you need to know to solve these sort of problems is that:
    [tex] i=\sqrt{-1} [/tex]
    [tex] i^2 = -1[/tex]
    [tex] i^3 = -i [/tex]
    [tex] i^4 = 1 [/tex] and the cycle repeats

    so to take your question:
    [tex] (-7+2i)(-10-3i) [/tex]
    [tex] 70+21i-20i-6i^2 [/tex]
    [tex] 76+i [/tex]

    you did in fact do it correctly.
  8. Nov 29, 2004 #7
    hey, for that latex all i have 2 do is put [-code-] the code and end. Correct?
    let me try.
    seems i have 2 put latex
    Last edited: Nov 29, 2004
  9. Nov 29, 2004 #8
    Thanks for taking the time to look it up, and thanks everyone who told me the first one is right what a relief! :eek: For simplifying (-2-5i) / by (3+4i) I got (-26-7i)/25 as my final answer can someone tell me is this right? :frown:
    Last edited: Nov 29, 2004
  10. Nov 30, 2004 #9
    yes you are right again
    (-2-5i)/(3+4i) does equal (-26-7i)/25
  11. Nov 30, 2004 #10

    Thanks sooo much! :tongue2: If anyone has any objections they may still speak :smile:
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