# Kenneth Rosen Edition 6th Exercise 5.5 Question 15 (Page No. 380)

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How many solutions are there to the equation
x1 + x2 + x3 + x4 + x5 = 21,
where xi , i = 1, 2, 3, 4, 5, is a nonnegative integer such that: 0$\leq$ x1$\leq$10 ?

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answer is 11,649 but don't know how...
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Nirmal Gaur  . please check bound condition. is it correct ? or, just $x_1$

$\left ( 0\leq x_i \leq 10 \right )$

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yes, it's right...(x1 not xi)...it's question no. 15 on page no. 432 of rosen(7th edition)...

x1 + x2 + x3 + x4 + x5 = 21, where xi , i = 1, 2, 3, 4, 5, is a nonnegative integer such that: 0≤ x1 ≤10 and x2,x3,x4,x5 can range in between 0 to 21.

Now we have to choose the coefficient of x21 in following expression

(1+ x + x2 +x3.....+ x10) (1+ x + x2 +x3.......+X21)(1+ x + x2 +x3.....+ x21)(1+ x + x2 +x3.....+ x21)(1+ x + x2+x3.....+ x21)

also we can extend term of x2,x3,x4,x5 from x21 upto infinty because it didn't  change the value of x21 coefficient .

(1+ x + x2 +x3.....+ x10) (1+ x + x2 +x3.......)(1+ x + x2 +x3.....+ )(1+ x + x2 +x3.....+ )(1+ x + x2+x3.....+ )

$\left ( \frac{1-x^{11}}{1-x} \right )\left ( \frac{1}{(1-x)^{4}} \right )$

$\left ( 1-x^{11} \right )\left ( \frac{1}{(1-x)^{5}} \right )$

now we have to find only coefficient of x21 for which we consider coefficient of x21 and x10 in $\left ( 1-x \right )^{-5}$

general term of binomial coefficient is $\binom{n+r-1}{r}$

$\binom{5+21-1}{21}-\binom{5+10-1}{10}$=12650-1001=11649

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great explaination sir...thanx a lot :) :)

x1 + x2 + x3 + x4 + x5 = 21, where xi , i = 1, 2, 3, 4, 5, is a nonnegative integer such that: 0≤ x1 ≤10 .

Which can be solved by formula :  $\binom{25}{4} - \binom{14}{4}$

Total possibilites without upper Bound : $\binom{25}{4}$

possibilites with upper Bound: $\binom{14}{4}$

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May be :)
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its not 15c4 , its 14c4
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condition will be like this:

xi>=0, So 25c4

we will remove all solutions where x1>=11 and xi>=0

so x1 + x2 +x3 +x4 +x5 =21

make x1>=0  by removing 11 from 21

x1 +x2 + x3 +x4 +x5 =10

(10 + 5-1 )c(5-1)

check this
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Thank you for pointing . Agreed. :)
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i was solving for x1+x2+x3+x4+x5=11, silly mistake... thanx for helping :)

x1 + x2 + x3 + x4 + x5 = 21

0≤x1 ≤3  ,1≤ x2<4  and x3≥15

case 1: x3≥15 given to make condition <x1, x2, x3, x4, x5> ≥ 0 remove 15 from 21.

so new equation becomes x1 + x2 + x3 + x4 + x5 = 21-15 =6

case 2: 1≤ x2<4=1≤ x2≤3 given so make it 0≤ x2 ≤2   for that remove 1 from 6.

so new equation becomes x1 + x2 + x3 + x4 + x5 =6-1=5

find all solutions where <x1, x2, x3, x4, x5> ≥ 0 lets say it solution A.

and remove the solution where 0≤ x2 ≤2 that is we will remove all solutions from A where x2>2 and all other variables are xi>=0 let's say it solution B.

0≤x1 ≤3 that is we will remove all solution from A where x1>3 and all other variables are xi>=0 let's say it solution C.

A= (5+5-1)C (5-1)

= 126

C= 5C4 =5 solutions

B=6C4 =15 solutions

B and C =0 solutions

Answer = A -  (B+C - B and C)

=126-20

=106

edited by

x1 + x2 + x3 + x4 + x5 = 21, where xi , i = 1, 2, 3, 4, 5, is a nonnegative integer such that: 0≤ x1 ≤10 ,0≤ x2 ≤10 ,0≤ x3 ≤10 ,0≤ x4 ≤10 ,0≤ x5 ≤10 ,

Solved by Generating Function :

(1+ x + x2 +x3.....+ x10) (1+ x + x2 +x3.....+ x10)(1+ x + x2 +x3.....+ x10)(1+ x + x2 +x3.....+ x10)(1+ x + x2 +x3.....+ x10) = x21.

((1+ x + x2 +x3.....+ x10))5 = x21.

$\left ( \frac{1- x^{11}}{1-x} \right )^{5}$ = x21.

$\left ( 1- x^{11} \right )^{5}\left ( \frac{1}{1-x} \right )^{5}$ =  x21.

$\left ( 1- x^{11} \right )^{5}\sum_{r=0}^{n}\binom{n+r-1}{r} x^{r} = x^{21}.$

$\left ( 1- x^{11} \right )^{5}\sum_{r=0}^{n}\binom{5+r-1}{r} x^{r} = x^{21}.$

$\left ( 1- x^{11} \right )^{5}\sum_{r=0}^{n}\binom{r+4}{r} x^{r} = x^{21}.$

Now we have to find term of x whose cofficient is 21.

$\left (\binom{5}{0} x^{0} - \binom{5}{1}x^{11} + \binom{5}{2}x^{22} - \binom{5}{3}x^{33}+ \binom{5}{4}x^{44} - \binom{5}{5}x^{55} \right )$  $\times \sum_{r=0}^{n}\binom{r+4}{r} x^{r} = x^{21}.$

$\left ( _{0}^{5}\textrm{C} \times _{21}^{25}\textrm{C}x^{21} \right ) - \left ( _{1}^{5}\textrm{C} \times _{10}^{14}\textrm{C}x^{21} \right ) = x^{21}$

$x^{21}\left ( 1 \times \left ( 25\times 23\times 22 \right ) \right ) - \left ( 5 \times\left ( 7\times 13\times 11 \right ) \right ) = x^{21}$

= 7645

1 vote

\begin{align*} &\Rightarrow x_1 + x_2 + x_3 + x_4 + x_5 = 21 \quad \left ( 0\leq x_i \leq 10 \right ) \\ \\ &\Rightarrow \text{No of integral solution of the above equation is } \\ &\Rightarrow {\color{red}{\left [ x^{21} \right ]}} \left ( 1+x+x^2+ \dots + x^{10} \right )^5 \\ &\Rightarrow {\color{red}{\left [ x^{21} \right ]}} \left [ \frac{1-x^{11}}{1-x} \right ]^5 \\ &\Rightarrow {\color{red}{\left [ x^{21} \right ]}} \left [ 1-x^{11} \right ]^5 \cdot \left [ \frac{1}{\left ( 1-x \right )^5} \right ] \\ &\Rightarrow {\color{red}{\left [ x^{21} \right ]}} \left [ \sum_{m=0}^{5}\binom{5}{m}\left ( -1 \right )^m \cdot x^{11\cdot m} \right ] \cdot \left [ \sum_{r=0}^{\infty}\binom{5+r-1}{r}\cdot x^r \right ] \\ &\Rightarrow {\color{red}{\left [ x^{21} \right ]}} \begin{cases} &+1 \cdot \binom{25}{21} \qquad m = 0,r = 21 \\ \\ &-5 \cdot \binom{14}{10} \qquad m = 1,r = 10 \\ \end{cases} \\ &\Rightarrow {\color{red}{\left [ x^{21} \right ]}} = +1 \cdot \binom{25}{21} -5 \cdot \binom{14}{10} = {\bf \color{red}{7645}} \\ \end{align*}

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but constraint is for x1 that it lies btw 0 and 10.why r u taking it for all??

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