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Math Help - checking my answer with another answer...

  1. #1
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    Exclamation checking my answer with another answer...

    im having problem with this

    (2+3i) (5-7i)
    the answer the teacher got is
    31+i

    i got 31-34i

    what did i do wrong?

    Thanks,
    candy
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  2. #2
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    Quote Originally Posted by Candy101 View Post
    im having problem with this

    (2+3i) (5-7i)
    the answer the teacher got is
    31+i

    i got 31-34i

    what did i do wrong?

    Thanks,
    candy
    Since you didn't include your working it's impossible to know what you did wrong. However, you would be advised to note that -14i + 15i = i.
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  3. #3
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    oh so sorry

    first i multiple out
    (2+3i) (5-7i)
    and i got

    10-14i-20i-21i^2

    so i know that i^2 is -1 so ... 21i^2 became 21
    giving me
    10-14i-20i+21

    i then combine -14i with -20i giving me -34i
    then i added 10 with 21 giving me 31

    thanks,
    candy
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  4. #4
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    Quote Originally Posted by Candy101 View Post
    oh so sorry

    first i multiple out
    (2+3i) (5-7i)
    and i got

    10-14i-20i-21i^2 ... there is your mistake. 3i(5) = 15i
    ...
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  5. #5
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    omg i see it! gosh thanks!
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