# Thread: Binomial Expansion Problem - Please help!

1. ## Binomial Expansion Problem - Please help!

Hey all this is my first problem i have posted

here is the question

In the binomial expansion of (2k+x)^n, where k is a constant and n is a positive integer, the coefficient of x^2 is equal to the coefficient of x^3

a) Prove that n=6k+2

Thanks, i really do not know where to start

thanks

2. Originally Posted by acid_rainbow
Hey all this is my first problem i have posted

here is the question

In the binomial expansion of (2k+x)^n, where k is a constant and n is a positive integer, the coefficient of x^2 is equal to the coefficient of x^3

a) Prove that n=6k+2

Thanks, i really do not know where to start

thanks
Start by writting down the two coefficients in question:

Coeff of $x^2$ : ${n \choose 2}(2k)^{n-2}$

Coeff of $x^3$ : ${n \choose 3}(2k)^{n-3}$

So you have:

${n \choose 2}(2k)^{n-2}= {n \choose 3}(2k)^{n-3}$

RonL

3. yea i got that far, but how do i solve/ rearrange that to get n=6k+2

4. how can you simplify something if its NC2 :SSSSS

so confused

5. Originally Posted by CaptainBlack
Start by writting down the two coefficients in question:

Coeff of $x^2$ : ${n \choose 2}(2k)^{n-2}$

Coeff of $x^3$ : ${n \choose 3}(2k)^{n-3}$

So you have:

${n \choose 2} (2k)^{n-2} = {n \choose 3}(2k)^{n-3}$

RonL
$
\frac{n!}{2!(n-2)!} \frac{(2k)^{n-2}}{(2k)^{n-3}}=\frac{n!}{3!(n-3)!}
$

so:

$
\frac{1}{2!(n-2)!} (2k) = \frac{1}{3!(n-3)!}
$

$
\frac{1}{(n-2)} (2k) = \frac{1}{3}
$

which will rearrange into what you want

RonL

6. thanks, sorry for double post, im not sure where the 1 comes from but i will just learn it.