k n = ( 2 ) · ( 5 ) = ( 7 ) · (2 – 7 ) for k k-r Prove Newton's Identity: for n ≥k≥r≥ 0.....

Advanced Engineering Mathematics
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ISBN:9780470458365
Author:Erwin Kreyszig
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Chapter2: Second-order Linear Odes
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**Problem 9: Prove Newton's Identity**

Prove the identity:

\[
\binom{n}{k} \cdot \binom{k}{r} = \binom{n}{r} \cdot \binom{n-r}{k-r}
\]

where \( n \geq k \geq r \geq 0 \).

---

**Explanation:**

- **\(\binom{n}{k}\):** This notation represents a binomial coefficient, defined as the number of ways to choose \( k \) elements from a set of \( n \) elements without regard to the order. It is calculated using the formula \(\frac{n!}{k!(n-k)!}\).

- **\(\binom{k}{r}\):** Similarly, this represents the number of ways to choose \( r \) elements from a set of \( k \) elements.

- The identity states that the product of these two binomial coefficients is equal to the product of \(\binom{n}{r}\) and \(\binom{n-r}{k-r}\).

The conditions \( n \geq k \geq r \geq 0 \) ensure that all binomial coefficients are well-defined (non-negative integers). Proving this identity involves combinatorial arguments or algebraic manipulations using properties of factorials.
Transcribed Image Text:**Problem 9: Prove Newton's Identity** Prove the identity: \[ \binom{n}{k} \cdot \binom{k}{r} = \binom{n}{r} \cdot \binom{n-r}{k-r} \] where \( n \geq k \geq r \geq 0 \). --- **Explanation:** - **\(\binom{n}{k}\):** This notation represents a binomial coefficient, defined as the number of ways to choose \( k \) elements from a set of \( n \) elements without regard to the order. It is calculated using the formula \(\frac{n!}{k!(n-k)!}\). - **\(\binom{k}{r}\):** Similarly, this represents the number of ways to choose \( r \) elements from a set of \( k \) elements. - The identity states that the product of these two binomial coefficients is equal to the product of \(\binom{n}{r}\) and \(\binom{n-r}{k-r}\). The conditions \( n \geq k \geq r \geq 0 \) ensure that all binomial coefficients are well-defined (non-negative integers). Proving this identity involves combinatorial arguments or algebraic manipulations using properties of factorials.
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