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    How do I re-arrange correctly a^3*b + b^3*a? So I can work out the values involving (a+b) and (ab).

    I know a^3 + b^3 = (A+B)^3 - 3ab(a+b), but I can't re-arrange this one for some reason. I've done a ton of them without problems, this one has tripped me up lol I need your help, really don't want to stifle my revision over this question.
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    Notice that a^3*b + a*b^3 = ab*(a^2 + b^2).
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    (Original post by generalebriety)
    Notice that a^3*b + a*b^3 = ab*(a^2 + b^2).
    I used that, and it came out with the wrong answer.
    Just when I thought I was getting the hang of this.

    Just so you know, a+b=-3 and ab=9

    Oh, from reading your thing, I think I recognised my mistake, will try it again. lol

    Cheers

    Aah, just reading what you wrote helped me work it out. I didn't work out the bit in the brackets first, so I got a bit stuck. I got -81 in the end, which is the right answer. Let me give you some rep.
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    you know that a + b = -3 and ab = 9

    and you know that a^2 + b^2 = (a + b)^2 - 2ab

    so you could use ab*[(a+b)^2 - 2ab]?

    So its -81
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    (Original post by Weeman5872)
    you know that a + b = -3 and ab = 9

    and you know that a^2 + b^2 = (a + b)^2 - 2ab

    so you could use ab*[(a+b)^2 - 2ab]?

    So its -81
    Lol I got the answer, but thanks anyways.
 
 
 
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Updated: February 14, 2010
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