# Binomial Expansion

• Dec 13th 2010, 08:36 AM
dd86
Binomial Expansion
Good Day,

I've been faced with a funny binomial expansion. Attached it with the answer from the book for your reference.

My question is, how do we get to the answer if I expanded (1-2x)^-2 first and then multiplied it with cos x?
• Dec 13th 2010, 09:12 AM
FernandoRevilla
Quote:

Originally Posted by dd86
My question is, how do we get to the answer if I expanded (1-2x)^-2 first and then multiplied it with cos x?

Yes,

$(1-2x)^{-2}\cos x=(1+2x+4x^2+8x^3+\ldots)^2(1-x^2/2!+\ldots)=$

$\ldots=1+4x+23x^2/2+30x^3+\ldots$

Fernando Revilla
• Dec 13th 2010, 09:37 AM
chisigma
Quote:

Originally Posted by dd86
Good Day,

I've been faced with a funny binomial expansion. Attached it with the answer from the book for your reference.

My question is, how do we get to the answer if I expanded (1-2x)^-2 first and then multiplied it with cos x?

Your approach is fully correct!... the series expansion of the term $\displaystyle \frac{1}{(1-2 x)^{2}}$ is obtained with the following steps...

$\displaystyle \frac{1}{(1-2 x)^{2}} = \frac{1}{2}\ \frac{d}{d x} \frac{1}{1-2 x} = \frac{1}{2}\ \frac{d}{d x} (1 + 2 x + 4 x^{2} + 8 x^{3} + 16 x^{4} + ...)=$

$\displaystyle = \frac{1}{2}\ (2 + 8 x + 24 x^{2} + 64 x^{3} + ...)= 1 + 4 x + 12 x^{2} + 32 x^{3} + ...$ (1)

http://digilander.libero.it/luposaba...ato&#91;1].jpg

Merry Christmas from Italy

$\chi$ $\sigma$
• Dec 13th 2010, 05:02 PM
dd86
Wow OK. I did not come across this in my self-study so far...

So what this means is that when I multiply a binomial expansion with cos x, I have to differentiate the binomial expansion?

What if it was cos 2x or cos3x or cos -x? Do I differentiate that many times for positive integers and integrate that many times for negative ones? What if it was cos ax if a is not an integer?

What about sin ax and tan ax?

It would be great if anyone could just give me the link to read up on this. I'm still a novice so something that's easy to understand would be preferable. But I will post questions if I don't understand :p
• Dec 13th 2010, 05:19 PM
dd86
I'd also like to know whether I can read up on how to find the rth term in a binomial expansion? I have found myself to be weak in this part for many of the questions I have come across.

Is this covered in a different topic in math?

• Dec 13th 2010, 06:23 PM
TheCoffeeMachine
The series expansion of $(1-2x)^{-2}$ can also be found this way:

$\displaystyle \frac{1}{(1-2x)^2} = \frac{1}{1-[1-(1-2x)^2]} = \frac{1}{1-t}$

$\displaystyle \sum_{k \ge 0}t^k = \sum_{k \ge 0}\left[1-(1-2x)^2\right]^k = \sum_{k\ge 0}4^kx^k(1-x)^k.$

@ dd86:
Spoiler:

Quote:

Originally Posted by dd86
Wow OK. I did not come across this in my self-study so far...

Geometric/Taylor/Maclaurin series, that's what you need for this problem.
Quote:

So what this means is that when I multiply a binomial expansion with cos x, I have to differentiate the binomial expansion?
No, you replace $\cos{x}$ with its Maclaurin expansion, which is:

$\displaystyle \cos x = \sum^{\infin}_{n=0} \frac{(-1)^n}{(2n)!} x^{2n} = 1 - \frac{x^2}{2!} + \frac{x^4}{4!} - \cdots$

Quote:

What if it was cos 2x or cos3x or cos -x?
Then you change each of those with their equivalent Maclaurin expansions.
Quote:

What about sin ax and tan ax?
Same for those too.
Quote:

It would be great if anyone could just give me the link to read up on this. I'm still a novice so something that's easy to understand would be preferable. But I will post questions if I don't understand :p
Probably wikipedia can't be bettered in this case.

Quote:

I'd also like to know whether I can read up on how to find the rth term in a binomial expansion? I have found myself to be weak in this part for many of the questions I have come across.
You only need the factorial definition of the coefficients and the binomial theorem to do that. Do a bit of search to find further explanations.
• Dec 17th 2010, 11:26 PM
dd86
Thank you, The CoffeMachine. Your tip with finding the rth term was very helpful. Had alotta fun in solving the questions I had left out before.
• Dec 18th 2010, 04:27 AM
HallsofIvy
Quote:

Originally Posted by dd86
Wow OK. I did not come across this in my self-study so far...

So what this means is that when I multiply a binomial expansion with cos x, I have to differentiate the binomial expansion?

No, he didn't say that. Chisigma did not use the binomial expansion, he used the derivatives to find the Taylor's series for $(1- 2x^2)^{-2}$. Of course, "a power series is a power series" so both methods give the same result.

Quote:

What if it was cos 2x or cos3x or cos -x? Do I differentiate that many times for positive integers and integrate that many times for negative ones? What if it was cos ax if a is not an integer?

What about sin ax and tan ax?

It would be great if anyone could just give me the link to read up on this. I'm still a novice so something that's easy to understand would be preferable. But I will post questions if I don't understand :p