Problem 16 - Algebra and Trigonometry

Question

Find all rational zeros of the polynomial, and write the polynomial in factored form. $$ P(x)=x^{3}-7 x^{2}+14 x-8 $$

Step-by-Step Solution

Answer

The rational zeros are 1, 2, and 4. The polynomial factors are \( (x-1)(x-2)(x-4) \).

Step 1: Identify Possible Rational Zeros

Use the Rational Root Theorem to identify possible rational zeros of the polynomial. For \( P(x) = x^3 - 7x^2 + 14x - 8 \), the potential rational zeros are the factors of the constant term divided by the factors of the leading coefficient.- Factors of the constant term (-8): ±1, ±2, ±4, ±8.- Factors of the leading coefficient (1): ±1.Thus, potential rational zeros are ±1, ±2, ±4, ±8.

Step 2: Test Possible Rational Zeros

Use synthetic division or direct substitution to test each possible rational zero identified in Step 1.- Test \( x = 1 \): \( P(1) = 1^3 - 7 \times 1^2 + 14 \times 1 - 8 = 0 \). Hence, \( x = 1 \) is a zero.- Test other possibilities (such as 2, -1, -2) similarly to confirm additional zeros, if any.Testing shows that \( x = 1 \), \( x = 2 \), and \( x = 4 \) all yield zero.

Step 3: Factor the Polynomial Using Found Zeros

Since you found that 1, 2, and 4 are zeros, these can be used to factor the polynomial.The polynomial can be written as:\[ P(x) = (x - 1)(x - 2)(x - 4) \]

Key Concepts

Rational Root Theorem

The Rational Root Theorem is a handy tool for unraveling potential rational zeros of a polynomial. It saves us from unnecessary trials by giving a list of potential candidates based on the polynomial's coefficients. Here's how it works:

Take the polynomial we are working with: \[ P(x) = x^3 - 7x^2 + 14x - 8 \]
This theorem tells us that any rational zero of this polynomial, expressed as \( \frac{p}{q} \), must fulfill a simple condition:

• \( p \) is a factor of the constant term (in this case, \(-8\))
• \( q \) is a factor of the leading coefficient (which here is \(1\) for \(x^3\))

Now, the factors of \(-8\) are \(\pm1, \pm2, \pm4, \pm8\) and the factors of \(1\) are simply \(\pm1\). So, by the Rational Root Theorem, our potential rational roots are \(\pm1, \pm2, \pm4, \pm8\). Having this list sets the stage for our next step—testing each candidate.

Polynomial Factoring

Once we identify the rational roots of a polynomial, we can use them to factor the polynomial into smaller, simpler factors. Factoring breaks the polynomial into parts that, when multiplied together, give the original polynomial.

For the polynomial \( P(x) = x^3 - 7x^2 + 14x - 8 \), we found the rational roots to be 1, 2, and 4 (i.e., values of \(x\) that satisfy \(P(x) = 0\)).
These zeros translate directly into factors:

• If \(x=1\) is a zero, \(x-1\) is a factor.
• If \(x=2\) is a zero, \(x-2\) is a factor.
• If \(x=4\) is a zero, \(x-4\) is a factor.

Therefore, the polynomial \( P(x) \) can be expressed as the product of these factors:\[P(x) = (x - 1)(x - 2)(x - 4)\]By factoring in this way, not only do we simplify the problem at hand, but we also gain insight into the nature and behavior of the polynomial under different conditions.

Synthetic Division

Synthetic division is a streamlined version of polynomial division. It's particularly useful when verifying potential rational roots. It helps determine if our candidate guess is indeed a root without the exhausting calculations of traditional division.

Here’s a brief guide on how to apply synthetic division to our polynomial \( P(x) = x^3 - 7x^2 + 14x - 8 \):

• Write down the coefficients of the polynomial: \(1, -7, 14, -8\).
• Choose a potential zero to test, such as \(x = 1\), and place it outside the synthetic division bracket.
• Bring down the leading coefficient as is.
• Multiply it by \(x\) and add it to the next coefficient underneath.
• Repeat the process across all the coefficients.

Once completed:- If the final number (remainder) is 0, \(x = 1\) is indeed a root.
- If it's not 0, then \(x = 1\) is not a root.

For our polynomial, synthetic division confirmed that \(x = 1, 2,\) and \(4\) are roots because each resulted in a remainder of 0. This concise method not only confirms roots but also assists in efficiently breaking down the polynomial into the factors discovered in previous steps.