Finding b_n - Binomial theorem problem

In summary: You can set $x=2$ now$$\sum_{r=n}^{2n} \frac{r!}{(r-n)!} a_r = \sum_{r=n}^{2n} \frac{r!}{(r-n)!} b_r$$$$\sum_{r=n}^{2n} \frac{r!}{(r-n)!} a_r = \sum_{r=n}^{2n} \frac{r!}{(r
  • #1
Saitama
4,243
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Question:
If $\displaystyle \sum_{r=0}^{2n} a_r(x-2)^r=\sum_{r=0}^{2n} b_r(x-3)^r$ and $a_k=1$ for all $k \geq n$, then show that $b_n={}^{2n+1}C_{n+1}$.

Attempt:
I haven't been able to make any useful attempt on this one. I could rewrite it to:

$$\sum_{r=0}^{n-1} a_r(x-2)^r + (x-2)^n\left(\frac{(x-2)^{n+1}-1}{(x-2)-1}\right)=\sum_{r=0}^{2n} b_r(x-3)^r$$

I am clueless about the next step.

Any help is appreciated. Thanks!
 
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  • #2
Pranav said:
Question:
If $\displaystyle \sum_{r=0}^{2n} a_r(x-2)^r=\sum_{r=0}^{2n} b_r(x-3)^r$ and $a_k=1$ for all $k \geq n$, then show that $b_n={}^{2n+1}C_{n+1}$.

Attempt:
I haven't been able to make any useful attempt on this one. I could rewrite it to:

$$\sum_{r=0}^{n-1} a_r(x-2)^r + (x-2)^n\left(\frac{(x-2)^{n+1}-1}{(x-2)-1}\right)=\sum_{r=0}^{2n} b_r(x-3)^r$$

I am clueless about the next step.

Any help is appreciated. Thanks!

Hi Pranav!

Did you already expand it for $n=2$ and $n=3$?
Just to see if we can find a pattern?

I don't have a solution yet, but I can see that $b_n$ can be fully deduced from the terms starting from n up to 2n.
All terms below n are irrelevant.Let us define $c_t$ such that the sum is equal to \(\displaystyle \sum_{t=0}^{2n} c_t x^t\).
That is, we have that:
$$\sum_{t=0}^{2n} c_t x^t = \sum_{r=0}^{2n} (x-2)^r=\sum_{s=0}^{2n} b_s(x-3)^s$$

With $a_{2n}=1$, we get that highest power $x^{2n}$ has coefficient:
$$c_{2n}=b_{2n}=a_{2n}=1$$
For $n=0$ we would be done, since $b_n = b_0 = \binom{2\cdot 0 + 1}{0+1} = \binom{1}{1} = 1$.Next is $c_{2n-1}$, which yields:
\begin{array}{lclcl}
c_{2n-1} &=& a_{2n-1} + \binom{2n}{1}a_{2n}(-2) &=& b_{2n-1} + \binom{2n}{1}b_{2n}(-3) \\
c_{2n-1} &=& 1 - 4n &=& b_{2n-1} - 6n \\
b_{2n-1} &=& 2n + 1
\end{array}
For $n=1$ we would be done, since $b_n = b_1 = 2\cdot 1 + 1 = \binom{2\cdot 1 + 1}{1+1} = \binom{3}{2} = 3$.Next is $c_{2n-2}$...
Sorry, that's as far as I got at this time. ;)
 
  • #3
I like Serena said:
Hi Pranav!

Did you already expand it for $n=2$ and $n=3$?
Just to see if we can find a pattern?

I don't have a solution yet, but I can see that $b_n$ can be fully deduced from the terms starting from n up to 2n.
All terms below n are irrelevant.Let us define $c_t$ such that the sum is equal to \(\displaystyle \sum_{t=0}^{2n} c_t x^t\).
That is, we have that:
$$\sum_{t=0}^{2n} c_t x^t = \sum_{r=0}^{2n} (x-2)^r=\sum_{s=0}^{2n} b_s(x-3)^s$$

With $a_{2n}=1$, we get that highest power $x^{2n}$ has coefficient:
$$c_{2n}=b_{2n}=a_{2n}=1$$
For $n=0$ we would be done, since $b_n = b_0 = \binom{2\cdot 0 + 1}{0+1} = \binom{1}{1} = 1$.Next is $c_{2n-1}$, which yields:
\begin{array}{lclcl}
c_{2n-1} &=& a_{2n-1} + \binom{2n}{1}a_{2n}(-2) &=& b_{2n-1} + \binom{2n}{1}b_{2n}(-3) \\
c_{2n-1} &=& 1 - 4n &=& b_{2n-1} - 6n \\
b_{2n-1} &=& 2n + 1
\end{array}
For $n=1$ we would be done, since $b_n = b_1 = 2\cdot 1 + 1 = \binom{2\cdot 1 + 1}{1+1} = \binom{3}{2} = 3$.Next is $c_{2n-2}$...
Sorry, that's as far as I got at this time. ;)

Thanks for your input ILS! :)

But is there any proper way of doing it, I mean by some algebraic manipulation? I am thinking of differentiating both the sides wrt r number of times. I will see if I reach somewhere using that.
 
  • #4
I have reached the right answer but I cannot perform a summation I come across midway in the solution.

Differentiating wrt r n times as I said before, I get:
$$\sum_{r=0}^{2n} r(r-1)...(r-n+1)a_r(x-2)^{r-n}=\sum_{r=0}^{2n} r(r-1)...(r-n+1)b_r(x-3)^{r-n}$$
$$\Rightarrow \sum_{r=0}^{2n} \frac{r!}{(r-n)!}a_r(x-2)^{r-n}=\sum_{r=0}^{2n} \frac{r!}{(r-n)!}b_r(x-3)^{r-n}$$
Substituting x=3 and using the fact that $a_k=1$ for all $k\geq n$,
$$\sum_{r=n}^{2n} \frac{r!}{(r-n)!}=n! \cdot b_n$$

I did the summation using Wolfram Alpha but how do I solve the summation manually? :confused:
 
  • #5
Pranav said:
I have reached the right answer but I cannot perform a summation I come across midway in the solution.

Differentiating wrt r n times as I said before, I get:
$$\sum_{r=0}^{2n} r(r-1)...(r-n+1)a_r(x-2)^{r-n}=\sum_{r=0}^{2n} r(r-1)...(r-n+1)b_r(x-3)^{r-n}$$
$$\Rightarrow \sum_{r=0}^{2n} \frac{r!}{(r-n)!}a_r(x-2)^{r-n}=\sum_{r=0}^{2n} \frac{r!}{(r-n)!}b_r(x-3)^{r-n}$$
Substituting x=3 and using the fact that $a_k=1$ for all $k\geq n$,
$$\sum_{r=n}^{2n} \frac{r!}{(r-n)!}=n! \cdot b_n$$

I did the summation using Wolfram Alpha but how do I solve the summation manually? :confused:

Nope. I have no clue.
Looks like you're nicely going in the right direction, seeing how everything collapses.

Btw, your summations should start at n instead of 0 after taking the nth derivative.

Anyway, I can see that the summation can be reduced to a summation of combinations that add up to another combination.
And I can see that it holds true for a number of examples.
But as yet I have no clue how to prove it.
 
  • #6
I like Serena said:
Nope. I have no clue.
Looks like you're nicely going in the right direction, seeing how everything collapses.

Btw, your summations should start at n instead of 0 after taking the nth derivative.

Anyway, I can see that the summation can be reduced to a summation of combinations that add up to another combination.
And I can see that it holds true for a number of examples.
But as yet I have no clue how to prove it.

Sorry for the late reply.

Is there any alternative method if there is no way to solve the summation?

Thanks!
 
  • #7
Pranav said:
Differentiating wrt r n times as I said before, I get:
$$\sum_{r=0}^{2n} r(r-1)...(r-n+1)a_r(x-2)^{r-n}=\sum_{r=0}^{2n} r(r-1)...(r-n+1)b_r(x-3)^{r-n}$$

You have some typos

You are differentiating with respect to $x$ .

The nth derivative with respect to $x$

$$\sum_{r=n}^{2n} r(r-1)...(r-n+1)a_r(x-2)^{r-n}=\sum_{r=n}^{2n} r(r-1)...(r-n+1)b_r(x-3)^{r-n}$$
 
  • #8
\(\displaystyle \sum_{r=n}^{2n} \frac{r!}{(r-n)!}=n! \cdot b_n\)

\(\displaystyle b_n =\sum_{r=n}^{2n} \frac{r!}{(r-n)!\, n! } \)

\(\displaystyle b_n =\sum_{r=n}^{2n} {n \choose r } \)

we need to prove that

\(\displaystyle \sum_{r=n}^{2n} {r \choose n } = {2n+1 \choose n+1}={2n+1 \choose n}\)

By induction on $n$

for $n = 1$ we have

\(\displaystyle \sum_{r=1}^{2} {r\choose 1 } = {3 \choose 1}\)

Next assume

\(\displaystyle \sum_{r=k}^{2k} {r \choose k } = {2k+1 \choose k}\)

and we need to prove that

\(\displaystyle \sum_{r=k+1}^{2k+2} {r \choose k+1 } = {2k+3 \choose k+1}\)

I will leave the rest for you.
 
  • #9
Given $\displaystyle \sum_{r=0}^{2n}a_{r}\left(x-2\right)^r = \sum_{r=0}^{2n}b_{r}\left(x-3\right)^r$ and $a_{k} = 1$ forall $k\geq n$

Let $(x-3) = t$ Then $(x-2) = (t+1)$

$\displaystyle \sum_{r=0}^{2n}a_{r}(1+t)^r = \sum_{r=0}^{2n}b_{r}t^r$

$\displaystyle a_{0}+a_{1}(1+t)+a_{2}(1+t)^2+...+a_{n}(1+t)^n+a_{n+1}(1+t)^{n+1}+...+a_{2n}(1+t)^{2n} = b_{0}+b_{1}t+b_{2}t^2+...+b_{n}t^n+b_{n+1}t^{n+1}+...+b_{2n}t^{2n}$

Now camparing Coefficient of $t^n$ on both side and using $a_{k}=1$ forall $k\geq n$

$\displaystyle \binom{n}{n}+\binom{n+1}{n}+\binom{n+2}{n}+...+\binom{2n}{n} = b_{n}$

$\displaystyle \binom{n+1}{n+1}+\binom{n+1}{n}+\binom{n+2}{n}+...+\binom{2n}{n} = b_{n}$

Now using $\displaystyle \binom{n}{r}+\binom{n}{r-1} = \binom{n+1}{r}$ succesively

$\displaystyle \binom{2n+1}{n+1} = b_{n}$

Here $\displaystyle b_{n} = $ Coefficient of $t^n$ on $\bf{R.H.S}$
 
  • #10
ZaidAlyafey said:
Next assume

\(\displaystyle \sum_{r=k}^{2k} {r \choose k } = {2k+1 \choose k}\)

and we need to prove that

\(\displaystyle \sum_{r=k+1}^{2k+2} {r \choose k+1 } = {2k+3 \choose k+1}\)

I will leave the rest for you.

This is where I got stuck...

However, the approach of jacks looks perfect! :)
 
  • #11
I like Serena said:
This is where I got stuck...

Agreed , I should have tried it before posting , sorry .
 
  • #12
jacks said:
Given $\displaystyle \sum_{r=0}^{2n}a_{r}\left(x-2\right)^r = \sum_{r=0}^{2n}b_{r}\left(x-3\right)^r$ and $a_{k} = 1$ forall $k\geq n$

Let $(x-3) = t$ Then $(x-2) = (t+1)$

$\displaystyle \sum_{r=0}^{2n}a_{r}(1+t)^r = \sum_{r=0}^{2n}b_{r}t^r$

$\displaystyle a_{0}+a_{1}(1+t)+a_{2}(1+t)^2+...+a_{n}(1+t)^n+a_{n+1}(1+t)^{n+1}+...+a_{2n}(1+t)^{2n} = b_{0}+b_{1}t+b_{2}t^2+...+b_{n}t^n+b_{n+1}t^{n+1}+...+b_{2n}t^{2n}$

Now camparing Coefficient of $t^n$ on both side and using $a_{k}=1$ forall $k\geq n$

$\displaystyle \binom{n}{n}+\binom{n+1}{n}+\binom{n+2}{n}+...+\binom{2n}{n} = b_{n}$

$\displaystyle \binom{n+1}{n+1}+\binom{n+1}{n}+\binom{n+2}{n}+...+\binom{2n}{n} = b_{n}$

Now using $\displaystyle \binom{n}{r}+\binom{n}{r-1} = \binom{n+1}{r}$ succesively

$\displaystyle \binom{2n+1}{n+1} = b_{n}$

Here $\displaystyle b_{n} = $ Coefficient of $t^n$ on $\bf{R.H.S}$

Thanks a lot jacks! :)

BTW, my friend came up with exactly the same procedure this morning and I was going to post it here as soon as I reach home.

EDIT: Looks like you made it a bit lengthy. Rewrite the summation as:

$$\sum_{r=0}^{n-1} a_r(1+t)^{r}+ (1+t)^n\frac{(1+t)^{n+1}-1}{t}=\sum_{r=0}^{2n}b_rt^r$$
$$\sum_{r=0}^{n-1} a_r(1+t)^{r}+ \frac{(1+t)^{2n+1}-(1+t)^n}{t}=\sum_{r=0}^{2n}b_rt^r$$
Now we need the coefficient of $t^n$ on both the sides.

On the left hand side, we find the coefficient of t^(n+1) from (1+t)^(2n+1) as there is a $t$ in the denominator.

Hence, the coefficient of t^n on LHS is $\binom{2n+1}{n+1}$.
 
Last edited:

FAQ: Finding b_n - Binomial theorem problem

What is the binomial theorem?

The binomial theorem is a mathematical formula that describes the expansion of a binomial expression raised to a positive integer power. It is used to simplify and solve problems involving binomial expressions.

How do you use the binomial theorem to find b_n?

To find b_n, you can use the formula b_n = nCk * a^(n-k) * b^k, where n is the power, k is the term number, a is the coefficient of the first term, and b is the coefficient of the second term. Simply plug in the given values and solve for b_n.

Can the binomial theorem be used for other types of expressions besides binomials?

Yes, the binomial theorem can be used for other types of expressions such as polynomials or trinomials. The formula remains the same, but the coefficients and exponents may vary.

What is the significance of b_n in the binomial theorem?

b_n represents the coefficient of the kth term in the binomial expansion. It is used to determine the value of the kth term in the expansion and is an important factor in solving problems involving binomial expressions.

Can the binomial theorem be extended to negative or fractional powers?

Yes, the binomial theorem can be extended to negative or fractional powers using the generalized binomial theorem formula, which includes the binomial coefficient and negative or fractional exponents. This allows for solving more complex problems involving binomial expressions.

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