Applying any method and show all the steps to determine whether f,(x) = cos 2x f,(x) = 1 f,(x)= cos x are linearly independent.

Advanced Engineering Mathematics
10th Edition
ISBN:9780470458365
Author:Erwin Kreyszig
Publisher:Erwin Kreyszig
Chapter2: Second-order Linear Odes
Section: Chapter Questions
Problem 1RQ
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**Problem Statement:**

Determine whether the functions \( f_1(x) = \cos 2x \), \( f_2(x) = 1 \), and \( f_3(x) = \cos^2 x \) are linearly independent by applying any method. Show all steps clearly.

---

**Steps for Determining Linear Independence:**

A set of functions \(\{f_1, f_2, f_3\}\) is said to be linearly independent if the equation:

\[ c_1 f_1(x) + c_2 f_2(x) + c_3 f_3(x) = 0 \]

holds true only when \( c_1 = c_2 = c_3 = 0 \) for all \(x\).

To determine this, we need to solve:

\[ c_1 \cos 2x + c_2 \cdot 1 + c_3 \cos^2 x = 0 \]

for all possible values of \(x\).

---

**Solving the Equation:**

1. Use trigonometric identities:
   - Recall the identity: \(\cos 2x = 2\cos^2 x - 1\)

2. Substitute \(\cos 2x\) in the equation:
   \[
   c_1 (2\cos^2 x - 1) + c_2 + c_3 \cos^2 x = 0
   \]

3. Rearrange terms:
   \[
   (2c_1 + c_3)\cos^2 x - c_1 + c_2 = 0
   \]

This equation must hold for all \(x\).

---

**Analyzing Coefficients:**

- Check if the coefficients of \(\cos^2 x\) and the constant terms can be zero independently:
   1. \(2c_1 + c_3 = 0\)
   2. \(-c_1 + c_2 = 0\)

- Solve the system of equations:

  From equation (1):
  \[
  c_3 = -2c_1
  \]

  From equation (2):
  \[
  c_2 = c_1
  \]

Therefore, substitute back to check for non-trivial solutions:

If \( c_1 = 0 \), then \(
Transcribed Image Text:**Problem Statement:** Determine whether the functions \( f_1(x) = \cos 2x \), \( f_2(x) = 1 \), and \( f_3(x) = \cos^2 x \) are linearly independent by applying any method. Show all steps clearly. --- **Steps for Determining Linear Independence:** A set of functions \(\{f_1, f_2, f_3\}\) is said to be linearly independent if the equation: \[ c_1 f_1(x) + c_2 f_2(x) + c_3 f_3(x) = 0 \] holds true only when \( c_1 = c_2 = c_3 = 0 \) for all \(x\). To determine this, we need to solve: \[ c_1 \cos 2x + c_2 \cdot 1 + c_3 \cos^2 x = 0 \] for all possible values of \(x\). --- **Solving the Equation:** 1. Use trigonometric identities: - Recall the identity: \(\cos 2x = 2\cos^2 x - 1\) 2. Substitute \(\cos 2x\) in the equation: \[ c_1 (2\cos^2 x - 1) + c_2 + c_3 \cos^2 x = 0 \] 3. Rearrange terms: \[ (2c_1 + c_3)\cos^2 x - c_1 + c_2 = 0 \] This equation must hold for all \(x\). --- **Analyzing Coefficients:** - Check if the coefficients of \(\cos^2 x\) and the constant terms can be zero independently: 1. \(2c_1 + c_3 = 0\) 2. \(-c_1 + c_2 = 0\) - Solve the system of equations: From equation (1): \[ c_3 = -2c_1 \] From equation (2): \[ c_2 = c_1 \] Therefore, substitute back to check for non-trivial solutions: If \( c_1 = 0 \), then \(
Expert Solution
Step 1

Here, use the method of Wronskian according to which Wronskian is non zero for linearly independent functions and wronskian of n functions is given by the following determinant:

 

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