If ${}^{9}{{P}_{5}}+5{}^{9}{{P}_{4}}={}^{10}{{P}_{r}}$ then find the value of r.
Answer
360.9k+ views
Hint: Use the fact that ${}^{n}{{P}_{r}}=\dfrac{n!}{\left( n-r \right)!}$ and $0\le r\le n$. Calculate the values of ${}^{9}{{P}_{5}}$ and ${}^{9}{{P}_{4}}$ ,hence determine the value of LHS. Now start by substituting r = 0,1,2,3, … till LHS = RHS.
Complete step-by-step answer:
$\begin{align}
& {}^{9}{{P}_{5}}=\dfrac{9!}{\left( 9-5 \right)!}=\dfrac{9!}{4!}=\dfrac{9\times 8\times 7\times 6\times 5\times 4!}{4!}=9\times 8\times 7\times 6\times 5=15120 \\
& {}^{9}{{P}_{4}}=\dfrac{9!}{\left( 9-4 \right)!}=\dfrac{9!}{5!}=\dfrac{9\times 8\times 7\times 6\times 5!}{5!}=9\times 8\times 7\times 6=3024 \\
\end{align}$
Hence LHS $=15120+5\times 3024=30240$.
When r = 0, RHS = $\dfrac{10!}{\left( 10-0 \right)!}=1$
When r = 1, RHS = 10
When r = 2 RHS = $10\times 9=90$
When r = 3 RHS = $90\times 8=720$
When r = 4 RHS = $720\times 7=5040$
When r = 5 RHS = $5040\times 6=30240$
Hence, we have when r = 5 LHS = RHS.
Hence r = 5.
Note: Proof by argument:
Number of Permutations of 10 Letters taken 5 at a time = Number of permutations if a particular letter (say A) always occupies the first position + Number of permutations if that particular letter never occupies the first position.
LHS = \[{}^{10}{{P}_{5}}\].
The number of permutations if a particular letter (say A) always occupies the first position = Number of permutations of remaining 9 letter taken 4 at a time = \[{}^{9}{{P}_{4}}\]
The number of permutations if a particular letter never occupies the first position = \[9\times {}^{9}{{P}_{4}}\] Because the first place has 9 choices [NOT TAKING THE PARTICULAR LETTER] and the remaining letters can be arranged in \[{}^{9}{{P}_{4}}\] ways.
Now we know that $\left( n-r+1 \right){}^{n}{{P}_{r-1}}={}^{n}{{P}_{r}}$
Hence, we have
$\begin{align}
& \left( 9-5+1 \right){}^{9}{{P}_{4}}={}^{9}{{P}_{5}} \\
& \Rightarrow 5{}^{9}{{P}_{4}}={}^{9}{{P}_{5}} \\
\end{align}$
Hence \[9{}^{9}{{P}_{4}}=5{}^{9}{{P}_{4}}+4{}^{9}{{P}_{4}}={}^{9}{{P}_{5}}+4{}^{9}{{P}_{4}}\]
Hence, we have
\[\begin{align}
& {}^{10}{{P}_{5}}={}^{9}{{P}_{4}}+{}^{9}{{P}_{5}}+4{}^{9}{{P}_{4}} \\
& \Rightarrow {}^{10}{{P}_{5}}={}^{9}{{P}_{5}}+5{}^{9}{{P}_{4}} \\
\end{align}\]
Complete step-by-step answer:
$\begin{align}
& {}^{9}{{P}_{5}}=\dfrac{9!}{\left( 9-5 \right)!}=\dfrac{9!}{4!}=\dfrac{9\times 8\times 7\times 6\times 5\times 4!}{4!}=9\times 8\times 7\times 6\times 5=15120 \\
& {}^{9}{{P}_{4}}=\dfrac{9!}{\left( 9-4 \right)!}=\dfrac{9!}{5!}=\dfrac{9\times 8\times 7\times 6\times 5!}{5!}=9\times 8\times 7\times 6=3024 \\
\end{align}$
Hence LHS $=15120+5\times 3024=30240$.
When r = 0, RHS = $\dfrac{10!}{\left( 10-0 \right)!}=1$
When r = 1, RHS = 10
When r = 2 RHS = $10\times 9=90$
When r = 3 RHS = $90\times 8=720$
When r = 4 RHS = $720\times 7=5040$
When r = 5 RHS = $5040\times 6=30240$
Hence, we have when r = 5 LHS = RHS.
Hence r = 5.
Note: Proof by argument:
Number of Permutations of 10 Letters taken 5 at a time = Number of permutations if a particular letter (say A) always occupies the first position + Number of permutations if that particular letter never occupies the first position.
LHS = \[{}^{10}{{P}_{5}}\].
The number of permutations if a particular letter (say A) always occupies the first position = Number of permutations of remaining 9 letter taken 4 at a time = \[{}^{9}{{P}_{4}}\]
The number of permutations if a particular letter never occupies the first position = \[9\times {}^{9}{{P}_{4}}\] Because the first place has 9 choices [NOT TAKING THE PARTICULAR LETTER] and the remaining letters can be arranged in \[{}^{9}{{P}_{4}}\] ways.
Now we know that $\left( n-r+1 \right){}^{n}{{P}_{r-1}}={}^{n}{{P}_{r}}$
Hence, we have
$\begin{align}
& \left( 9-5+1 \right){}^{9}{{P}_{4}}={}^{9}{{P}_{5}} \\
& \Rightarrow 5{}^{9}{{P}_{4}}={}^{9}{{P}_{5}} \\
\end{align}$
Hence \[9{}^{9}{{P}_{4}}=5{}^{9}{{P}_{4}}+4{}^{9}{{P}_{4}}={}^{9}{{P}_{5}}+4{}^{9}{{P}_{4}}\]
Hence, we have
\[\begin{align}
& {}^{10}{{P}_{5}}={}^{9}{{P}_{4}}+{}^{9}{{P}_{5}}+4{}^{9}{{P}_{4}} \\
& \Rightarrow {}^{10}{{P}_{5}}={}^{9}{{P}_{5}}+5{}^{9}{{P}_{4}} \\
\end{align}\]
Last updated date: 29th Sep 2023
•
Total views: 360.9k
•
Views today: 3.60k
Recently Updated Pages
What is the Full Form of DNA and RNA

What are the Difference Between Acute and Chronic Disease

Difference Between Communicable and Non-Communicable

What is Nutrition Explain Diff Type of Nutrition ?

What is the Function of Digestive Enzymes

What is the Full Form of 1.DPT 2.DDT 3.BCG

Trending doubts
How do you solve x2 11x + 28 0 using the quadratic class 10 maths CBSE

Difference between Prokaryotic cell and Eukaryotic class 11 biology CBSE

Summary of the poem Where the Mind is Without Fear class 8 english CBSE

Difference Between Plant Cell and Animal Cell

What is the basic unit of classification class 11 biology CBSE

Fill the blanks with the suitable prepositions 1 The class 9 english CBSE

One cusec is equal to how many liters class 8 maths CBSE

Differentiate between homogeneous and heterogeneous class 12 chemistry CBSE

Give 10 examples for herbs , shrubs , climbers , creepers
