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# Binomial expansion and approximation question

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1. 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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2. (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.

(ii) means that

So just solve then plug this value of into your expansion. But you're still off by a factor of something so you'll need to multiply your answer by that.
3. (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)?
4. (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.

(ii) means that

So just solve then plug this value of into your expansion.
Why is it 28/8 instead of 28?

Spoiler:
Show
and see, beat me again
5. (Original post by KaylaB)
Why is it 28/8 instead of 28?
Spoiler:
Show
and see, beat me again
Doesn't count because the last bit of what I sad was wrong!

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

It should be , 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
7. (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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