### The Quotient Rule: AP Calculus AB/BC Study Guide

#### Introduction

Hello, future calculus champions! Ready to slice and dice some functions? Buckle up because today we’re diving into the Quotient Rule, your new best friend for dealing with some of the trickiest derivatives out there. Think of the Quotient Rule as the superhero cape you don when basic differentiation just won’t cut it. 🦸♂️📉

#### The Quotient Rule: Breaking It Down

When it comes to differentiating one function divided by another, the Quotient Rule steps in like a calculus wizard with a wand of wisdom. Here’s the magic spell, written in math language:

$$ \frac{d}{dx} \left[ \frac{f(x)}{g(x)} \right] = \frac{g(x) \frac{d}{dx} f(x) - f(x) \frac{d}{dx} g(x)}{[g(x)]^2} $$

If that looks like a formula only Dumbledore could love, don’t worry! It’s easier than it seems. Here’s the secret: rather than adding products of functions and their derivatives like the Product Rule, the Quotient Rule subtracts them and then divides everything by the square of the denominator. Easy peasy, right? 🍋

To make it simpler, suppose you have: $$ u = f(x) \quad \text{and} \quad v = g(x) $$

Then the magic formula transforms into: $$ \frac{d}{dx} \left[ \frac{u}{v} \right] = \frac{v \frac{d}{dx} u - u \frac{d}{dx} v}{v^2} $$

And voilà, you have the Quotient Rule in all its glory—it’s practically a piece of cake as long as both ( u ) and ( v ) are differentiable! 🎂

#### Quotient Rule in Action

Let’s sprinkle some mathematical fairy dust on an example and make it all sparkle! ✨

**Example 1:**

Let's find the derivative of: $$ y = \frac{x^2 + x - 2}{x^3 + 6} $$

Define ( f(x) ) and ( g(x) ) as follows: $$ f(x) = x^2 + x - 2 \quad \text{and} \quad g(x) = x^3 + 6 $$

The Quotient Rule tells us: $$ \frac{dy}{dx} = \frac{(x^3 + 6) \frac{d}{dx} (x^2 + x - 2) - (x^2 + x - 2) \frac{d}{dx} (x^3 + 6)}{(x^3 + 6)^2} $$

First, we need the derivatives: $$ \frac{d}{dx} (x^2 + x - 2) = 2x + 1 $$ $$ \frac{d}{dx} (x^3 + 6) = 3x^2 $$

Now plug them in: $$ \frac{dy}{dx} = \frac{(x^3 + 6)(2x + 1) - (x^2 + x - 2)(3x^2)}{(x^3 + 6)^2} $$

Simplify it by expanding and combining like terms: $$ \frac{dy}{dx} = \frac{(2x^4 + x^3 + 12x + 6) - (3x^4 + 3x^3 - 6x^2)}{(x^3 + 6)^2} $$

Combine and simplify: $$ \frac{dy}{dx} = \frac{-x^4 - 2x^3 + 6x^2 + 12x + 6}{(x^3 + 6)^2} $$

And there we have the derivative—differentiation mission accomplished! 🎯

#### Quotient Rule Examples: Fun with Functions

**Example 2: Exponential Fun**

Find the derivative of: $$ y = \frac{e^x}{1 + x^2} $$

Let ( f(x) = e^x ) and ( g(x) = 1 + x^2 ). We get their derivatives: $$ \frac{d}{dx} (e^x) = e^x \quad \text{and} \quad \frac{d}{dx} (1 + x^2) = 2x $$

Applying the Quotient Rule: $$ \frac{dy}{dx} = \frac{(1 + x^2)(e^x) - (e^x)(2x)}{(1 + x^2)^2} $$ $$ = \frac{e^x(1 + x^2 - 2x)}{(1 + x^2)^2} $$ $$ = \frac{e^x(1 - x)^2}{(1 + x^2)^2} $$

Voilà! Your exponential function is now properly differentiated. 🌟

**Example 3: Trigonometrical Delight**

Find the derivative of: $$ y = \frac{\sin(x)}{1 + \cos(x)} $$

Here, ( f(x) = \sin(x) ) and ( g(x) = 1 + \cos(x) ). The derivatives are: $$ \frac{d}{dx} (\sin(x)) = \cos(x) \quad \text{and} \quad \frac{d}{dx} (\cos(x)) = -\sin(x) $$

Thus, $$ \frac{dy}{dx} = \frac{(1 + \cos(x))(\cos(x)) - (\sin(x))(-\sin(x))}{(1 + \cos(x))^2} $$ $$ = \frac{\cos(x)(1 + \cos(x)) + \sin^2(x)}{(1 + \cos(x))^2} $$ $$ = \frac{\sin(x)(1 + \cos(x) - \sin(x))}{(1 + \cos(x))^2} $$

And voilà! You’ve got your differentiated trig function. 🌞

#### Key Terms to Remember

**Chain Rule:** A derivative technique for composite functions, which states that the derivative of a composite function is the derivative of the outer function times the derivative of the inner function.

**Denominator:** The bottom part of a fraction that represents the total number of equal parts into which a whole is divided.

**Function:** A relationship where each input value (domain) corresponds to exactly one output value (range).

**Quotient Rule:** A rule for finding the derivative of a quotient of two functions, stating that the derivative of a fraction is the denominator times the derivative of the numerator minus the numerator times the derivative of the denominator, all divided by the square of the denominator.

#### Conclusion

Fantastic job! Using the Quotient Rule, you've turned complex differentiation problems into manageable tasks. 🎉 So next time you face a division of functions, just remember—you’ve got the magic formula at your fingertips. Now go forth and conquer those derivatives like a calculus superhero! 🦸♀️📏

Good luck, and may your functions always be differentiable!