Problem 28 - Beginning and Intermediate Algebra

Question

Factor completely. $$5 r^{3}+40 r^{2}+80 r$$

Step-by-Step Solution

Answer

The completely factored expression is \(5r(r + 4)^2\).

Step 1: Identify the common factor among all terms

Observe each term in the expression \(5r^3 + 40r^2 + 80r\). Note that every term has a factor of 5 and r in common. The greatest common factor (GCF) for these terms, in this case, is \(5r\).

Step 2: Divide the expression by the GCF

Divide each term of the given expression by the GCF to get a new expression: \[\frac{5r^3 + 40r^2 + 80r}{5r} = r^2 + 8r + 16\]

Step 3: Factor the resulting quadratic expression

We now need to factor the quadratic expression \(r^2 + 8r + 16\). Find two numbers that multiply to 16 and add to 8. Those numbers are 4 and 4. So, we can rewrite the quadratic expression as: \(r^2 + 8r + 16 = (r + 4)(r + 4)\)

Step 4: Write the fully factored expression

Now that we have factored the quadratic expression, we can insert it back into our expression, along with the GCF, to get the final factored expression: \(5r^3 + 40r^2 + 80r = 5r(r + 4)(r + 4)\) To make it more concise, we can write the final factored expression as: \(5r^3 + 40r^2 + 80r = 5r(r + 4)^2\)

Key Concepts

Greatest Common Factor

The greatest common factor (GCF) is an essential concept in factoring polynomials. It refers to the largest factor that divides each term of the polynomial without leaving a remainder. Identifying the GCF is the first step towards simplifying polynomials correctly.

When we deal with expressions like the one in our example, \(5r^3 + 40r^2 + 80r\), the first thing to do is identify all the factors common to each term. In this expression, each term contains both the number 5 and the variable \(r\). Hence, \(5r\) is the greatest common factor for the whole expression. Extracting this from the expression:

• For the term \(5r^3\), dividing by \(5r\) gives \(r^2\).
• For the term \(40r^2\), dividing by \(5r\) results in \(8r\).
• For the term \(80r\), dividing by \(5r\) gives \(16\).

After extracting the GCF, the expression becomes \(r^2 + 8r + 16\), making it simpler to factor further.

Quadratic Expression

Quadratic expressions are a common type of polynomial and have the general form \(ax^2 + bx + c\). The expression resulting from dividing by the GCF, \(r^2 + 8r + 16\), is a quadratic expression.

This specific quadratic is ready to be factored further. Factoring quadratics typically involves finding two numbers that multiply to the coefficient of the constant term and add to the coefficient of the linear term. In \(r^2 + 8r + 16\), we aim to find numbers that multiply to 16 (the constant) and add to 8 (the linear term's coefficient).

The numbers 4 and 4 work perfectly because:

• \(4 \times 4 = 16\)
• \(4 + 4 = 8\)

Therefore, the quadratic \(r^2 + 8r + 16\) can be expressed as \((r + 4)(r + 4)\) or simply \((r + 4)^2\). This reveals the structure of the polynomial and presents an opportunity for further simplification.

Factoring Techniques

Factoring techniques are strategies used to express a polynomial as the product of its factors. Identifying and applying the correct technique is crucial in arriving at the simplest form of an expression.

In the example \(5r^3 + 40r^2 + 80r\), once the GCF \(5r\) has been factored out, you are left with the quadratic \(r^2 + 8r + 16\). Factoring this involves the technique of finding pairs of numbers that satisfy both multiplication and addition conditions for the quadratic's constants.

After confirming that \((r + 4)^2\) is the factorization of \(r^2 + 8r + 16\), combine it with the GCF determined earlier. Thus, the final fully factored form of the polynomial is \(5r(r + 4)^2\).

Each technique - from extracting the GCF to factoring quadratic expressions - plays a distinctive role in simplifying polynomials. This structured approach ensures you precisely factor any given polynomial, paving the way towards a clearer understanding of polynomial arithmetic.