V5 + v6 Find a monic integral polynomial for which x is a root and prove that x is algebraically irrational.

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

Find a monic integral polynomial for which \( x \) is a root, where \( x = \sqrt{5} + \sqrt{6} \). Prove that \( x \) is algebraically irrational.

To solve this problem:

1. **Set Up the Expression:**

   Let \( x = \sqrt{5} + \sqrt{6} \).

2. **Square Both Sides:**

   \[
   x^2 = (\sqrt{5} + \sqrt{6})^2 = 5 + 2\sqrt{30} + 6 = 11 + 2\sqrt{30}
   \]

3. **Isolate the Square Root:**

   \[
   x^2 - 11 = 2\sqrt{30}
   \]

4. **Square Again to Eliminate the Square Root:**

   \[
   (x^2 - 11)^2 = (2\sqrt{30})^2 = 120
   \]

   \[
   x^4 - 22x^2 + 121 = 120
   \]

5. **Simplify:**

   \[
   x^4 - 22x^2 + 1 = 0
   \]

This polynomial is monic (leading coefficient is 1) and integral, and \( x \) is its root.

**Proof of Irrationality:**

Since \( x = \sqrt{5} + \sqrt{6} \) represents a sum of two irrational numbers, it cannot be expressed as a ratio of two integers, and therefore it is algebraically irrational. The polynomial \( x^4 - 22x^2 + 1 = 0 \) further confirms that \( x \) is a root that satisfies this condition.
Transcribed Image Text:**Problem Statement:** Find a monic integral polynomial for which \( x \) is a root, where \( x = \sqrt{5} + \sqrt{6} \). Prove that \( x \) is algebraically irrational. To solve this problem: 1. **Set Up the Expression:** Let \( x = \sqrt{5} + \sqrt{6} \). 2. **Square Both Sides:** \[ x^2 = (\sqrt{5} + \sqrt{6})^2 = 5 + 2\sqrt{30} + 6 = 11 + 2\sqrt{30} \] 3. **Isolate the Square Root:** \[ x^2 - 11 = 2\sqrt{30} \] 4. **Square Again to Eliminate the Square Root:** \[ (x^2 - 11)^2 = (2\sqrt{30})^2 = 120 \] \[ x^4 - 22x^2 + 121 = 120 \] 5. **Simplify:** \[ x^4 - 22x^2 + 1 = 0 \] This polynomial is monic (leading coefficient is 1) and integral, and \( x \) is its root. **Proof of Irrationality:** Since \( x = \sqrt{5} + \sqrt{6} \) represents a sum of two irrational numbers, it cannot be expressed as a ratio of two integers, and therefore it is algebraically irrational. The polynomial \( x^4 - 22x^2 + 1 = 0 \) further confirms that \( x \) is a root that satisfies this condition.
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