I How does this binomial expand?

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TL;DR Summary
I’m trying to expand 1/(1+𝑥)^2 and (1+𝑥)^-2
1/(1+x)2 and (1+x)-2 are the same but the negative index binomial has an infinite expansion, but the other as a denominator, stops at x 2

So please help me understand this
Thank you in advance.
 
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lioric said:
TL;DR Summary: I’m trying to expand 1/(1+𝑥)^2 and (1+𝑥)^-2

1/(1+x)2 and (1+x)-2 are the same but the negative index binomial has an infinite expansion, but the other as a denominator, stops at x 2

So please help me understand this
Thank you in advance.
$$\frac 1 {(1+x)^2} = \frac 1 {1 + 2x + x^2}$$That is not a power series in ##x##. That is the reciprocal of quadratic in ##x##. That function can also be expressed, using the Binomial Theorem, as an infinite power series in ##x##:
$$\frac 1 {(1+x)^2} = (1 + x)^{-2} = 1 -2x + 3x^2 - 4x^3 \dots$$Note, however, that this power series has a radius of convergence and is not valid for all ##x \in \mathbb R##.
 
Yes I know that. But what’s bugging me is, what if I do the reciprocal after expanding the bracket, them it’s not the same.
If I expand the (1+x)2 and i do a reciprocal,(which I still don’t know how) I don’t see how I could get the 1-2x+3x2-4x3…….
 
lioric said:
Yes I know that. But what’s bugging me is, what if I do the reciprocal after expanding the bracket, them it’s not the same.
If I expand the (1+x)2 and i do a reciprocal,(which I still don’t know how) I don’t see how I could get the 1-2x+3x2-4x3…….
$$(1 + (2x + x^2))^{-1} = 1 - (2x + x^2) + (2x + x^2)^2 - (2x + x^2)^3 \dots$$$$= 1 - 2x + 3x^2 -4x^3 \dots$$
 
Thank you for this. Could you explain a bit more how this form is derived? Im a bit unfamiliar with the (1+(2x+x2))-1 part came from.
 
lioric said:
Thank you for this. Could you explain a bit more how this form is derived? Im a bit unfamiliar with the (1+(2x+x2))-1 part came from.
That's a binomial expansion with ##2x+x^2## being the expansion variable (if that's the right term).
 
How did 1/(1+x)^2 Become that?
Could you use the example from the question and show this?
 
lioric said:
How did 1/(1+x)^2 Become that?
Could you use the example from the question and show this?
Ok I figured this out
But, what if you do it in reverse?
1-2x+3x^2-4x^3 and reciprocal that? How does the extra parts get removed from the infinit sequence to for a finite one?
My main problem is that wish to see the cancellation of those
 
lioric said:
Ok I figured this out
But, what if you do it in reverse?
1-2x+3x^2-4x^3 and reciprocal that? How does the extra parts get removed from the infinit sequence to for a finite one?
If ##A = B## then ##B = A##. You can do the binomial expansion in reverse:
$$1 - 2x + 3x^2 - 4x^3 + \dots = (1 + x)^{-2} = \frac 1{(1 + x)^2} = \frac 1 {1 + 2x + x^2}$$
 
  • #10
PeroK said:
If ##A = B## then ##B = A##. You can do the binomial expansion in reverse:
$$1 - 2x + 3x^2 - 4x^3 + \dots = (1 + x)^{-2} = \frac 1{(1 + x)^2} = \frac 1 {1 + 2x + x^2}$$
Can’t we reciprocal first and then factorize?
 
  • #11
lioric said:
Can’t we reciprocal first and then factorize?
You mean something like this?$$\frac 1 {1 - 2x + 3x^2 - 4x^3 + \dots} = \frac1 {(1 + x)^{-2}} = (1 + x)^2 = 1 + 2x + x^2$$
 
  • #12
PeroK said:
You mean something like this?$$\frac 1 {1 - 2x + 3x^2 - 4x^3 + \dots} = \frac1 {(1 + x)^{-2}} = (1 + x)^2 = 1 + 2x + x^2$$
Yeah but wouldn’t the powers become negetive when you flip them
 
  • #13
lioric said:
Yeah but wouldn’t the powers become negetive when you flip them
Not in the world of mathematics I inhabit!
 
  • #14
PeroK said:
Not in the world of mathematics I inhabit!
Could we do it this way ?
1/1-1-1/(2x)-1+1/(3x2)-1
 
  • #15
lioric said:
Could we do it this way ?
1/1-1-1/(2x)-1+1/(3x2)-1
I’m sorry I’m it didn’t make sense, but this is what I’m trying to figure out
 
  • #16
lioric said:
I’m sorry I’m it didn’t make sense, but this is what I’m trying to figure out
I can't help you with that.
 
  • #17
PeroK said:
I can't help you with that.
Ok. Thank you for all that you did. It did help.
 
  • #18
PeroK said:
I can't help you with that.
In conclusion what you are saying is that we can reciprocal the entire thing, but not separately?
 
  • #19
lioric said:
In conclusion what you are saying is that we can reciprocal the entire thing, but not separately?
This is elementary:
$$\frac 1{1 + x} \ne \frac 1 1 + \frac 1 x$$
 
  • #20
PeroK said:
This is elementary:
$$\frac 1{1 + x} \ne \frac 1 1 + \frac 1 x$$
Ok thank you
 
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