The Student Room Group

Binomial expansion

{2+x}^-2 how would you expand this in ascending powers of 1/x ?
Take x out of the bracket so it's x^-2 (1 +2/x)^-2 then expand that
Reply 2
Original post by Meowstic
Take x out of the bracket so it's x^-2 (1 +2/x)^-2 then expand that

How would you know what to factor out? Usually, I would take the 2 out and then have (1+1/2x)^-2. An explanation would be really helpful:smile:
Reply 3
Original post by Sidd1
{2+x}^-2 how would you expand this in ascending powers of 1/x ?

Do you mind posting a pic of the original question? Just because it's a bit unusual.
Original post by Sidd1
How would you know what to factor out? Usually, I would take the 2 out and then have (1+1/2x)^-2. An explanation would be really helpful:smile:

You want to expand in factors of 1/x so you make the term in the bracket into 1 + a(1/x)
Reply 5
Original post by Sir Cumference
Do you mind posting a pic of the original question? Just because it's a bit unusual.

Sure:smile: image.jpg
Reply 6
Original post by RDKGames
This question doesn't make sense at A-Level context. Binomial expansion is done about the point x=0x=0 at A-Level, so naturally no matter how you rewrite the end result, it will always be an expansion in increasing powers of xx, not 1x\frac{1}{x}.

Even if you 'take x out of the bracket' you will ultimately have a series that increases in powers of xx.

If this question genuinely wants you to obtain the expansion in increasing powers of 1x\frac{1}{x}, then the expansion needs to happen about the point x=x=\infty. This is way beyond A-Level and known as 'Laurent series'.

Umm okayyyyyyy that's weird.
Original post by Sidd1
Umm okayyyyyyy that's weird.


Oh ignore it, I overlooked something in the analysis. Factoring out x^(-2) is fine. Only issue I have with this question is that the (x+2)^(-2) is defined at x=0 but the expansion obviously isn't. So you should be careful when it comes to stating the region of validity of this expansion.
There's no issue with the question. It's perfectly fine.

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