# Need help for Binomial!

• Dec 19th 2012, 10:07 PM
Tutu
Need help for Binomial!
Hi I need help with these questions on binomial theorem!

1.) From the binomial expansion of $(1+x)^n$, deduce that
${2n+1 \choose 0}+{2n+1 \choose 1}+{2n+1 \choose 2}+...+{2n+1 \choose n}=4^n$

Using T( r+1) I know the coefficient of $x^n$ is ${2n+1 \choose n}$. I had let n in ${n \choose r}$ be 2n+1 and x be 1.
That way, I got $(2)^{2n+1}$ Is this wrong because from here, I do not know how to get $4^n$

2.) By considering the binomial expansion of $(1+x)^n$, find $\sum\limits_{r=0}^n2^r{n \choose r}$
I got $2^n$ as an answer for this one, I had just expanded out the summation and then let x=1. It's wrong..

Thank you so much!
• Dec 20th 2012, 01:11 AM
chiro
Re: Need help for Binomial!
Hey Tutu.

For 2) consider x = 2 instead of x = 1.

For 1) consider the binomial expansion (2 + 2)^n and how 4^n * [n]Cr gives [2n+1]Cr when you consider that adding all nCr's from 0 to n gives 2^n which means the square of this gives (2^n)^2 = 2^(2n) = (2^2)^n = 4^n.