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# C4 Binomial Expansion help Watch

1. I'm stuck on part b).
I have the expansion from part a) but do not understand why we can't just equate 8-9x to 7100 to work out x.

How do we know to take out a cube root of 100 before we find x? Why is this allowed but my method not?

2. (Original post by JLegion)

I'm stuck on part b).
I have the expansion from part a) but do not understand why we can't just equate 8-9x to 7100 to work out x.
Well, doing it that way, what value of x do you get?

And is it in the set of values where the expansion is valid. i.e. |x| < 8/9 ?

How do we know to take out a cube root of 100 before we find x? Why is this allowed but my method not?

cube root of 1000, not 100.

You need to take out an exact cube, whose value you know, to reduce what's under the cube root sign to allow a valid value for x.
3. http://www.thestudentroom.co.uk/show....php?t=4179073

4. (Original post by ghostwalker)
Well, doing it that way, what value of x do you get?

And is it in the set of values where the expansion is valid. i.e. |x| < 8/9 ?

cube root of 1000, not 100.

You need to take out an exact cube, whose value you know, to reduce what's under the cube root sign to allow a valid value for x.
100 was a typo.

Thank you for the help!
5. (Original post by Katiee224)
http://www.thestudentroom.co.uk/show....php?t=4179073

Thanks, that helps explain things further. Appreciate it

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