Skill: Differentiate quotients using the quotient rule

Explanation and Free Practice Resources

The quotient rule expresses the derivative of f(x)/g(x)f(x)/g(x), where ff and gg are differentiable and g(x)≠0g(x)\ne0, as g(x)f′(x)−f(x)g′(x)[g(x)]2\frac{g(x)f'(x)-f(x)g'(x)}{[g(x)]^2}. It captures how simultaneous changes in the numerator and denominator affect a quotient, including the importance of the subtraction order and the squared denominator; applications are limited to standard algebraic and transcendental functions rather than more advanced generalized derivative theories.

Detailed Explanation: Differentiate quotients using the quotient rule

To differentiate a quotient, use the quotient rule:

(f(x)g(x))′=g(x)f′(x)−f(x)g′(x)[g(x)]2,g(x)≠0.\left(\frac{f(x)}{g(x)}\right)' = \frac{g(x)f'(x)-f(x)g'(x)}{[g(x)]^2}, \qquad g(x)\ne 0.

A helpful way to remember the order is:

bottom times derivative of top minus top times derivative of bottom, all over bottom squared.

Example

Find the derivative of

y=x2+13x−2.y=\frac{x^2+1}{3x-2}.

Step 1: Identify the numerator and denominator

Let

f(x)=x2+1andg(x)=3x−2.f(x)=x^2+1 \qquad\text{and}\qquad g(x)=3x-2.

Step 2: Find their derivatives

f′(x)=2xandg′(x)=3.f'(x)=2x \qquad\text{and}\qquad g'(x)=3.

Step 3: Substitute into the quotient rule

y′=(3x−2)(2x)−(x2+1)(3)(3x−2)2.y' = \frac{(3x-2)(2x)-(x^2+1)(3)}{(3x-2)^2}.

Notice that the subtraction is important: it is

g(x)f′(x)−f(x)g′(x),g(x)f'(x)-f(x)g'(x),

not the other way around. Also, the entire denominator is squared.

Step 4: Simplify the numerator

(3x−2)(2x)−3(x2+1)=6x2−4x−3x2−3=3x2−4x−3.(3x-2)(2x)-3(x^2+1) = 6x^2-4x-3x^2-3 = 3x^2-4x-3.

Therefore,

y′=3x2−4x−3(3x−2)2.\boxed{y'=\frac{3x^2-4x-3}{(3x-2)^2}}.

The original function, and therefore its derivative formula, is defined only when 3x−2≠03x-2\ne0, so x≠23x\ne \frac23.

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