Binomial expansion: By writing (1+x)/(1-2x) as (1+x)(1-2x)^-1, show the cubic approximation of (1+x)/(1-2x) is 1 + 3x + 6x^2 + 12x^3. If anyone can help me out here at all. Im studying A level maths, and I am completely stumped by this question!
by 
mak
ANSWERS: 1
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I'll have a go. The generalised binomial is : (1 + X)^n = 1 + n X + n (n-1)/2! X^2 + n(n-1)(n-2)/3! X^3 + ... You have to learn that. so I'll do that for (1-2x)^-1 we'll get 1 + (-1) (2x) + (-1)(-1-1)/2! (2x)^2 + ... keep that going until you have ... + something * x^3 and stick "+ higher terms" on the end. Then multiply the whole thing by (1+x) I have done all this and I got "1 + 3x + 6x^2 + 12x^3 + higher terms"
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