Question 130 of 480
Solve for x in the equation x3 - 5x2 - x + 5 = 0
- A. 1, - 1, or 5
- B. 1, 1, or -5
- C. -1, 1, or -5
- D. 1, 1, or 5
Correct Answer:
A
Explanation
To solve the equation \( x^3 - 5x^2 - x + 5 = 0 \), we will use a combination of techniques including factoring and the Rational Root Theorem. Let's go through the solution step-by-step.
### Step 1: Identify the Polynomial
The equation we need to solve is:
\[
x^3 - 5x^2 - x + 5 = 0
\]
This is a cubic polynomial, and we are looking for its roots.
### Step 2: Use the Rational Root Theorem
The Rational Root Theorem suggests that any rational solution (root) of the polynomial equation, in the form \( \frac{p}{q} \), where \( p \) is a factor of the constant term (5) and \( q \) is a factor of the leading coefficient (1).
The factors of 5 are \( \pm 1, \pm 5 \). Since the leading coefficient is 1, the possible rational roots are:
\[
\pm 1, \pm 5
\]
### Step 3: Test Possible Roots
We will test these possible roots by substituting them into the polynomial.
1. **Testing \( x = 1 \)**:
\[
1^3 - 5(1^2) - 1 + 5 = 1 - 5 - 1 + 5 = 0
\]
So, \( x = 1 \) is a root.
2. **Testing \( x = -1 \)**:
\[
(-1)^3 - 5(-1)^2 - (-1) + 5 = -1 - 5 + 1 + 5 = 0
\]
So, \( x = -1 \) is also a root.
3. **Testing \( x = 5 \)**:
\[
5^3 - 5(5^2) - 5 + 5 = 125 - 125 - 5 + 5 = 0
\]
So, \( x = 5 \) is also a root.
### Step 4: Factor the Polynomial
Since we have found three roots \( x = 1, -1, 5 \), we can express the polynomial as:
\[
(x - 1)(x + 1)(x - 5) = 0
\]
### Step 5: Verify the Factorization
To ensure our factorization is correct, we can expand:
\[
(x - 1)(x + 1) = x^2 - 1
\]
Now multiply by \( (x - 5) \):
\[
(x^2 - 1)(x - 5) = x^3 - 5x^2 - x + 5
\]
This confirms our factorization is correct.
### Step 6: State the Roots
The roots of the equation \( x^3 - 5x^2 - x + 5 = 0 \) are:
\[
x = 1, -1, 5
\]
### Conclusion: Correct Option
The correct option is **A: 1, -1, or 5**.
### Explanation of Other Options
- **Option B: 1, 1, or -5**: This is incorrect because it suggests that 1 is a double root and includes -5, which is not a root.
- **Option C: -1, 1, or -5**: This is incorrect because it includes -5, which is not a root.
- **Option D: 1, 1, or 5**: This is incorrect because it suggests that 1 is a double root, which is not the case.
### Revision Summary
- The roots of the polynomial \( x^3 - 5x^2 - x + 5 = 0 \) are found using the Rational Root Theorem.
- The roots are \( x = 1, -1, 5 \).
- The polynomial can be factored as \( (x - 1)(x + 1)(x - 5) = 0 \).
- The correct answer is option A: 1, -1, or 5.