n (7) Vn € N, Σi!i = (n + 1)! — 1. i=1

Advanced Engineering Mathematics
10th Edition
ISBN:9780470458365
Author:Erwin Kreyszig
Publisher:Erwin Kreyszig
Chapter2: Second-order Linear Odes
Section: Chapter Questions
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Prove by induction.

The given mathematical formula expresses a summation identity for natural numbers:

\[
(7) \quad \forall n \in \mathbb{N}, \, \sum_{i=1}^{n} i \cdot i = (n+1)! - 1.
\]

This equation states that for any natural number \( n \), the sum of the squares of the numbers from 1 to \( n \) multiplied by their index \( i \) is equal to the factorial of \( n+1 \) minus 1.

### Explanation of Terms:

- **\(\forall n \in \mathbb{N}\)**: This denotes "for all \( n \)" where \( n \) is a member of the set of natural numbers.
- **\(\sum_{i=1}^{n} i \cdot i\)**: This is the summation notation, indicating that you sum over the index \( i \) from 1 to \( n \) where each term in the sum is the product of \( i \) and itself.
- **\((n+1)!\)**: This is the factorial of \( n+1 \), which means multiplying all positive integers up to \( n+1 \).
- **\(-1\)**: This indicates that 1 is subtracted from the result of the factorial.

This identity showcases a relationship between summation and factorials, providing insight into combinatorial mathematics and algebra.
Transcribed Image Text:The given mathematical formula expresses a summation identity for natural numbers: \[ (7) \quad \forall n \in \mathbb{N}, \, \sum_{i=1}^{n} i \cdot i = (n+1)! - 1. \] This equation states that for any natural number \( n \), the sum of the squares of the numbers from 1 to \( n \) multiplied by their index \( i \) is equal to the factorial of \( n+1 \) minus 1. ### Explanation of Terms: - **\(\forall n \in \mathbb{N}\)**: This denotes "for all \( n \)" where \( n \) is a member of the set of natural numbers. - **\(\sum_{i=1}^{n} i \cdot i\)**: This is the summation notation, indicating that you sum over the index \( i \) from 1 to \( n \) where each term in the sum is the product of \( i \) and itself. - **\((n+1)!\)**: This is the factorial of \( n+1 \), which means multiplying all positive integers up to \( n+1 \). - **\(-1\)**: This indicates that 1 is subtracted from the result of the factorial. This identity showcases a relationship between summation and factorials, providing insight into combinatorial mathematics and algebra.
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