Find the remainder when 122605 is divided by 17.

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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**Problem Statement:**

Find the remainder when \(12^{2605}\) is divided by 17.

---

**Explanation:**

To solve problems of this type, typically involving large powers and finding remainders, techniques such as Fermat's Little Theorem or Euler's Theorem may be useful.

**Key Concepts:**

- **Fermat's Little Theorem**: If \(p\) is a prime number and \(a\) is any integer not divisible by \(p\), then \(a^{p-1} \equiv 1 \pmod{p}\).

In this specific problem, since 17 is a prime number:
- According to Fermat's Little Theorem: 

  \[
  12^{16} \equiv 1 \pmod{17}
  \]

Thus, to find \(12^{2605} \mod 17\), first determine the power modulo 16 because of Fermat's Theorem:

\[
2605 \div 16 = 162 \text{ remainder } 13
\]

This means \(2605 \equiv 13 \pmod{16}\).

Therefore:

\[
12^{2605} \equiv 12^{13} \pmod{17}
\]

Calculating progressively:

\[
12^1 \equiv 12 \pmod{17}
\]
\[
12^2 \equiv 144 \equiv 8 \pmod{17}
\]
\[
12^4 \equiv (12^2)^2 \equiv 8^2 \equiv 64 \equiv 13 \pmod{17}
\]
\[
12^8 \equiv (12^4)^2 \equiv 13^2 \equiv 169 \equiv 16 \equiv -1 \pmod{17}
\]

Thus, \(12^{13} = 12^8 \cdot 12^4 \cdot 12 \equiv (-1) \cdot 13 \cdot 12\).

Continuing:

\[
(-1) \cdot 13 \equiv -13 \equiv 4 \pmod{17}
\]

\[
4 \cdot 12 \equiv 48 \equiv 14 \pmod{17}
\]

Thus, the remainder when \(12^{2605
Transcribed Image Text:**Problem Statement:** Find the remainder when \(12^{2605}\) is divided by 17. --- **Explanation:** To solve problems of this type, typically involving large powers and finding remainders, techniques such as Fermat's Little Theorem or Euler's Theorem may be useful. **Key Concepts:** - **Fermat's Little Theorem**: If \(p\) is a prime number and \(a\) is any integer not divisible by \(p\), then \(a^{p-1} \equiv 1 \pmod{p}\). In this specific problem, since 17 is a prime number: - According to Fermat's Little Theorem: \[ 12^{16} \equiv 1 \pmod{17} \] Thus, to find \(12^{2605} \mod 17\), first determine the power modulo 16 because of Fermat's Theorem: \[ 2605 \div 16 = 162 \text{ remainder } 13 \] This means \(2605 \equiv 13 \pmod{16}\). Therefore: \[ 12^{2605} \equiv 12^{13} \pmod{17} \] Calculating progressively: \[ 12^1 \equiv 12 \pmod{17} \] \[ 12^2 \equiv 144 \equiv 8 \pmod{17} \] \[ 12^4 \equiv (12^2)^2 \equiv 8^2 \equiv 64 \equiv 13 \pmod{17} \] \[ 12^8 \equiv (12^4)^2 \equiv 13^2 \equiv 169 \equiv 16 \equiv -1 \pmod{17} \] Thus, \(12^{13} = 12^8 \cdot 12^4 \cdot 12 \equiv (-1) \cdot 13 \cdot 12\). Continuing: \[ (-1) \cdot 13 \equiv -13 \equiv 4 \pmod{17} \] \[ 4 \cdot 12 \equiv 48 \equiv 14 \pmod{17} \] Thus, the remainder when \(12^{2605
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