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# Can't understand question in C4 June 2013 (R)

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1. I just don't get the question in C4 June 2013 (R) question 4b on binomial expansions.

I can't post it here but the mark scheme solution to the question is just bizarre.
If anyone can explain it to me it would be great.
2. (Original post by kc006)
I just don't get the question in C4 June 2013 (R) question 4b on binomial expansions.

I can't post it here but the mark scheme solution to the question is just bizarre.
If anyone can explain it to me it would be great.
note that cube root of 7100 = cube root of (1000 * 7.1) = 10 * cube root of 7.1
Now set 8 - 9x = 7.1 -> 9x = 0.9 -> x = 0.1.
Hence put 0.1 into your expansion from part (a) to get the approximation.
3. (Original post by HapaxOromenon3)
note that cube root of 7100 = cube root of (1000 * 7.1) = 10 * cube root of 7.1
Now set 8 - 9x = 7.1 -> 9x = 0.9 -> x = 0.1.
Hence put 0.1 into your expansion from part (a) to get the approximation.
wait isnt the cube root of (7100) the same as cube root of (71*100)? not sure why its a 7.1, am i missing something?
4. (Original post by kc006)
wait isnt the cube root of (7100) the same as cube root of (71*100)? not sure why its a 7.1, am i missing something?
71*100=7.1*1000
5. (Original post by AlmostNotable)
71*100=7.1*1000
jesus christ im screwed for tomorrow
6. (Original post by kc006)
wait isnt the cube root of (7100) the same as cube root of (71*100)? not sure why its a 7.1, am i missing something?

while youre right, technically

but that doesn't help answer the question. We need to get cube root of 7100 into a form where it can be substituted into the expansion. Also, our x needs to be less than 8/9
i.e. we need to write cube root (7100) in a similar form to cube root (8-9x)

while 7100 is 71*100, 100 is not a cube root. by dividing by taking out 1000 you can write it as 10*cube root (7.1)
We now need to make 8-9x=7.1
we get x =0.1
so,
cube root 7100 = 10[cube root(8-9x)] when x = 0.1

to answer the question, sub in 0.1 to your expansion that youve found, then multiply by 10

does that make sense?
7. (Original post by kc006)
jesus christ im screwed for tomorrow
honestly don't beat yourself up about it; also had a look at the markscheme and theres a typo there - it says 71 when it should say 7.1
Spoiler:
Show
but yes, i too am totally screwed. Im just so exhausted I want it all to be over
8. (Original post by timelizard)
honestly don't beat yourself up about it; also had a look at the markscheme and theres a typo there - it says 71 when it should say 7.1
Spoiler:
Show
but yes, i too am totally screwed. Im just so exhausted I want it all to be over
same here
9. If you know the expansion of 8-9x to the power of one third.
Substitute 0.1 for x and you have the cube-root of 7.1.
10. (Original post by kc006)
same here
hey, how did it go?

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