Problem 35 - Calculus Early Transcendentals: Pearson New International Edition

Question

Find \(D_{x} y\) using the rules of this section. $$ y=\frac{1}{4 x^{2}-3 x+9} $$

Step-by-Step Solution

Answer

The derivative is \( D_x y = -\frac{8x - 3}{(4x^2 - 3x + 9)^2} \).

Step 1: Identify the Differentiation Rule

The given function is a rational function of the form \( y = \frac{1}{u(x)} \). To differentiate it, we'll use the chain rule combined with the power rule.

Step 2: Rewrite the Function

Rewrite the given function \( y = \frac{1}{4x^2 - 3x + 9} \) as \( y = (4x^2 - 3x + 9)^{-1} \) to apply the power rule.

Step 3: Differentiate Using the Chain and Power Rules

To differentiate \( (4x^2 - 3x + 9)^{-1} \), apply the chain rule. Let \( u = 4x^2 - 3x + 9 \), then \( y = u^{-1} \). The derivative is \( D_x y = -1 \cdot u^{-2} \cdot D_x u \).

Step 4: Calculate the Derivative of \( u \)

Find \( D_x u = D_x (4x^2 - 3x + 9) = 8x - 3 \).

Step 5: Substitute Back and Simplify

Substitute \( D_x u = 8x - 3 \) and \( u = 4x^2 - 3x + 9 \) into the derivative formula \( D_x y = -1 \cdot u^{-2} \cdot D_x u \). So, \( D_x y = -\frac{8x - 3}{(4x^2 - 3x + 9)^2} \).

Key Concepts

Chain Rule

The chain rule is a fundamental concept in calculus used to differentiate composite functions. It allows us to break down a complex function into simpler parts, making differentiation manageable. If you have a function that is embedded within another function, you will need the chain rule.For example, consider a function \( y = (4x^2 - 3x + 9)^{-1} \), where the outer function is \( f(u) = u^{-1} \) and the inner function is \( u = 4x^2 - 3x + 9 \). To find the derivative, the chain rule tells us to firstly differentiate the outer function while keeping the inner function unchanged, and then multiply by the derivative of the inner function.

• Differentiate the outer function: \( f'(u) = -u^{-2} \).
• Don't forget to multiply by the derivative of the inner function: \( D_x u = 8x - 3 \).

By combining these derivatives, we achieve the formula: \( D_x y = -1 \cdot u^{-2} \cdot D_x u \), aiding us in handling more complicated formulations than just simple polynomials.

Power Rule

The power rule is a quick and effective way to find the derivative of functions with exponents in the form of \( x^n \). According to the power rule, if \( n \) is any real number, the derivative of \( x^n \) is \( nx^{n-1} \).When dealing with our exercise, where we rewrote \( y \) as \( (4x^2 - 3x + 9)^{-1} \), the entire expression can be treated through the power rule. The overall power of \( u \, (-1) \) dictates the application of the power rule:

• Differentiating \( u^{-1} \) gives us \(-1 \cdot u^{-2} \).

However, since \( u \) itself is a function of \( x \), we also use the chain rule alongside the power rule. This showcases how multiple differentiation approaches can work in tandem to simplify complex derivatives effectively.

Rational Functions

Rational functions are expressions made up of ratios of polynomials. In essence, they are what you can depict as \( \frac{P(x)}{Q(x)} \), where both \( P(x) \) and \( Q(x) \) are polynomial functions. The given exercise, \( y = \frac{1}{4x^2 - 3x + 9} \), is a clear example of a rational function.Differentiating rational functions involves understanding how to handle both the numerator and the denominator, especially when the denominator is a more complex polynomial. Using rules like the chain rule and the power rule becomes essential as they help navigate through the derivatives of both sections of the rational function.

• Identify the structure: it's easier to tackle once you recognize them as fractions.
• Simplify your work using established rules: equations like that in our exercise can be rewritten to fit these rules easier.

With rational functions, the combination of observation and calculated application of derivative rules allows us to comprehend and solve these expressions effectively.