Find a positive integer less than 8 for x that satisfies x = 720 + 3 (mod 7).

Advanced Engineering Mathematics
10th Edition
ISBN:9780470458365
Author:Erwin Kreyszig
Publisher:Erwin Kreyszig
Chapter2: Second-order Linear Odes
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**Problem:**

Find a positive integer less than 8 for \( x \) that satisfies the following congruence:

\[ x \equiv 7^{20} + 3 \pmod{7} \]

**Solution Explanation:**

1. **Understanding the Congruence:**
   \[
   x \equiv 7^{20} + 3 \pmod{7}
   \]
   This implies we are looking for the remainder of \( 7^{20} + 3 \) when divided by 7.

2. **Simplifying \( 7^{20} \mod 7 \):**
   Since \( 7 \equiv 0 \pmod{7} \), it follows that any power of 7 is also congruent to 0 modulo 7:
   \[
   7^{20} \equiv 0^{20} \equiv 0 \pmod{7}
   \]

3. **Adding 3:**
   Now substitute back into the equation:
   \[
   x \equiv 0 + 3 \equiv 3 \pmod{7}
   \]

4. **Conclusion:**
   The positive integer less than 8 that satisfies the original congruence is:
   \[
   x = 3
   \]

Thus, the solution is \( x = 3 \).
Transcribed Image Text:**Problem:** Find a positive integer less than 8 for \( x \) that satisfies the following congruence: \[ x \equiv 7^{20} + 3 \pmod{7} \] **Solution Explanation:** 1. **Understanding the Congruence:** \[ x \equiv 7^{20} + 3 \pmod{7} \] This implies we are looking for the remainder of \( 7^{20} + 3 \) when divided by 7. 2. **Simplifying \( 7^{20} \mod 7 \):** Since \( 7 \equiv 0 \pmod{7} \), it follows that any power of 7 is also congruent to 0 modulo 7: \[ 7^{20} \equiv 0^{20} \equiv 0 \pmod{7} \] 3. **Adding 3:** Now substitute back into the equation: \[ x \equiv 0 + 3 \equiv 3 \pmod{7} \] 4. **Conclusion:** The positive integer less than 8 that satisfies the original congruence is: \[ x = 3 \] Thus, the solution is \( x = 3 \).
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