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    Hey! Could some one point me in the right direction for part b. ii. ?
    Also if you spot any mistakes in my working please let me know : )
    Thanks!
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    (Original post by Matt Howard)
    Hey! Could some one point me in the right direction for part b. ii. ?
    Also if you spot any mistakes in my working please let me know : )
    Thanks!
    (a) All correct.

    (b) (i) All correct except for the last line. You're not allowed to willy-nilly multiply things like that.

    Your answer as \frac{1}{3} - \frac{2x}{81}  + \frac{8}{2187} x^2 is the correct and final (simplified) answer.

    (ii) 27 + 6x = 28 means that (27 + 6x)^{-1/3} = \left(28\right)^{-1/3} = \frac{1}{28^{1/3}}

    So just solve 27 +6x = 28 then plug this value of x into your expansion. But you're still off by a factor of something so you'll need to multiply your answer by that.
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    (Original post by Matt Howard)
    Hey! Could some one point me in the right direction for part b. ii. ?
    Also if you spot any mistakes in my working please let me know : )
    Thanks!
    If you've found the expansion for (27+6x)^(-1/3) which is that same as 1/(27+6x)^(1/3)
    And you're trying to find 2/28^(1/3)

    You could try making 28 = 27+6x and find a suitable value for x

    By substituting that value of x in to your expansion, this would give you 1/28^(1/3) so what would you need to do to get it to 2/28^(1/3)?
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    (Original post by Zacken)
    (a) All correct.

    (b) (i) All correct except for the last line. You're not allowed to willy-nilly multiply things like that.

    Your answer as \frac{1}{3} - \frac{2x}{81}  + \frac{8}{2187} x^2 is the correct and final (simplified) answer.

    (ii) 27 + 6x = \frac{28}{8} means that (27 + 6x)^{-1/3} = \left(\frac{28}{8}\right)^{-1/3} = \frac{2}{8^{1/3}}

    So just solve 27 +6x = \frac{28}{8} then plug this value of x into your expansion.
    Why is it 28/8 instead of 28?

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    (Original post by KaylaB)
    Why is it 28/8 instead of 28?
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    Doesn't count because the last bit of what I sad was wrong!

    It should be 28, I was rushing out for dinner when I posted what I did. Thanks. I'll correct it!
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    (Original post by Zacken)
    Doesn't count because the last bit of what I sad was wrong!

    It should be 28, I was rushing out for dinner when I posted what I did. Thanks. I'll correct it!
    This is what happens when you rush things
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    (Original post by KaylaB)
    This is what happens when you rush things
    Oh well, t'was a team effort, I pointed out a mistake in (i) you gave the correct approach to (ii).
 
 
 
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