Question

Simplify the expression. $$\frac{3 x}{x^{2}-2 x-15} \cdot(x+3)$$

Short answer

The simplified form of the given expression is \(\frac{3x}{x-5}\).

Step-by-step solution

Identify and Factor the Quadratic

First, observe the quadratic expression in the denominator, \(x^{2}-2 x-15\). This is factorable. With use of factorization method for quadratic equations, look for two numbers whose sum is -2 and whose product is -15 which are -5 and 3.\n Hence, factorize \(x^{2} - 2x - 15\) as \((x - 5)(x + 3)\). The expression then turns into \(\frac{3x}{(x-5)(x+3)} * (x+3)\).

Simplify the Expression

Now, \(x+3\) in the numerator and \((x + 3)\) in the denominator cancel each other out.\n\n Therefore, the simplified expression becomes \(\frac{3x}{x-5}\).

Key concepts

Factoring Quadratic Equations

When dealing with algebra, factoring quadratic equations is a fundamental skill necessary to simplify expressions. A quadratic equation typically takes the form of \(ax^2 + bx + c = 0\), where \(a\), \(b\), and \(c\) are constants and \(x\) represents the unknown variable.

To factor such an equation, one must find two binomials that when multiplied together produce the original quadratic. The process often involves finding two numbers that will both add up to the coefficient of the \(x\) term, and multiply to the constant term (to which we refer as the product-sum method).

For example, with the quadratic expression \(x^2 - 2x - 15\), we look for two numbers whose sum equals \(b = -2\) and whose product equals \(c = -15\), leading us to -5 and 3. We can then rewrite the expression as \(x - 5)(x + 3)\), breaking down the problem into simpler parts which can be easier to manage in subsequent steps of simplification.

Simplifying Rational Expressions

Simplifying rational expressions involves reducing the complexity of a fraction where both the numerator and the denominator are polynomials. To simplify such expressions, we look for common factors in the numerator and the denominator that can be divided out.

Consider the expression \(\frac{3x}{(x - 5)(x + 3)} \cdot (x + 3)\). Here, the term \(x + 3\) is present both in the numerator and the denominator. According to the fundamental property of fractions, if a term is present both above and below the fraction line, and is not zero, it can be cancelled out.

This cancellation simplifies the expression considerably, leading us from \(\frac{3x(x + 3)}{(x - 5)(x + 3)}\) to a much simpler \(\frac{3x}{x - 5}\), thus making the problem less intimidating and easier to understand.

Cancellation in Fractions

Cancellation in fractions is a method which significantly eases the simplification process. It is based on the principle that dividing the numerator and the denominator by the same non-zero factor does not change the value of the fraction.

In our example \(\frac{3x}{(x - 5)(x + 3)} \cdot (x + 3)\), cancelling the \(x + 3\) terms reduces the fraction to \(\frac{3x}{x - 5}\). It is important to ensure that the term we are cancelling is not equal to zero since division by zero is undefined.

Cancellation can greatly simplify the complexity of an algebraic expression, but we must apply the cancellation law correctly to avoid altering the expressions’ values. Remember to always look for common factors that appear in both the numerator and the denominator before attempting to cancel; this is a central tactic in simplifying rational expressions.