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# Binomial expansion watch

1. In the expansion of (1+x)^n, the expansion is valid provided IxI < 1,
and in the expansion of (1-2x)^-3, the expansion is valid for I2xI < 1, what does it mean?
2. I don't know if you need to know it for A-level maths but In order to get to that point, you have to get any numbers within the bracket to a 1. I can give an example:

(5+2x)^-1 = 5^-1 (1+2x/5)^-1 Therefore you would put |x|<5/2

If anything is wrong please reply and tell me so I can edit it so that they don't get the wrong idea.
3. (Original post by monkeyman0121)
I don't know if you need to know it for A-level maths but In order to get to that point, you have to get any numbers within the bracket to a 1. I can give an example:

(5+2x)^-1 = 5^-1 (1+2x/5)^-1 Therefore you would put |x|<5/2

If anything is wrong please reply and tell me so I can edit it so that they don't get the wrong idea.
Thanx... that's really help. Now, i understand.
4. (Original post by Nirza)
Thanx... that's really help. Now, i understand.
Another way of putting it is |x| has to be smaller than the other number in the bracket. (The process of what I did in the other comment made it easier to find the limits. Also, you must always get a 1 in the bracket.)

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