Skills Review for the Chain Rule

Learning Outcomes

  • Find the derivative of an implicit function by using implicit differentiation
  • Write the equation of a line using slope and a point on the line

In the Chain Rule section, we will learn how to apply the chain rule to functions of several variables. Here we will review implicit differentiation and how to write the equation of a line.

What is Implicit Differentiation?

Implicit differentiation allows us to find slopes of tangents to curves that are clearly not functions (they fail the vertical line test). We are using the idea that portions of [latex]y[/latex] are functions that satisfy the given equation, but that [latex]y[/latex] is not actually a function of [latex]x[/latex].

To perform implicit differentiation on an equation that defines a function [latex]y[/latex] implicitly in terms of a variable [latex]x[/latex], use the following steps:

  1. Take the derivative of both sides of the equation. Keep in mind that [latex]y[/latex] is a function of [latex]x[/latex]. Consequently, whereas [latex]\frac{d}{dx}(\sin x)= \cos x, \, \frac{d}{dx}(\sin y)= \cos y\frac{dy}{dx}[/latex] because we must use the Chain Rule to differentiate [latex]\sin y[/latex] with respect to [latex]x[/latex].
  2. Rewrite the equation so that all terms containing [latex]\frac{dy}{dx}[/latex] are on the left and all terms that do not contain [latex]\frac{dy}{dx}[/latex] are on the right.
  3. Factor out [latex]\frac{dy}{dx}[/latex] on the left.
  4. Solve for [latex]\frac{dy}{dx}[/latex] by dividing both sides of the equation by an appropriate algebraic expression.

Example: Using Implicit Differentiation

Assuming that [latex]y[/latex] is defined implicitly by the equation [latex]x^2+y^2=25[/latex], find [latex]\frac{dy}{dx}[/latex].

Example: Using Implicit Differentiation and the Product Rule

Assuming that [latex]y[/latex] is defined implicitly by the equation [latex]x^3 \sin y+y=4x+3[/latex], find [latex]\frac{dy}{dx}[/latex].

Try It

Find [latex]\frac{dy}{dx}[/latex] for [latex]y[/latex] defined implicitly by the equation [latex]4x^5+ \tan y=y^2+5x[/latex].

Try It

Write the Equation of a Line

(also in Module 1, Skills Review for Parametric Equations and Calculus of Parametric Equations)
The slope of a line refers to the ratio of the vertical change in y over the horizontal change in x between any two points on a line.

A General Note: The Slope of a Line

The slope of a line, m, represents the change in y over the change in x. Given two points, [latex]\left({x}_{1},{y}_{1}\right)[/latex] and [latex]\left({x}_{2},{y}_{2}\right)[/latex], the following formula determines the slope of a line containing these points:

[latex]m=\frac{{y}_{2}-{y}_{1}}{{x}_{2}-{x}_{1}}[/latex]

Example: Finding the Slope of a Line Given Two Points

Find the slope of a line that passes through the points [latex]\left(2,-1\right)[/latex] and [latex]\left(-5,3\right)[/latex].

Try It

Find the slope of the line that passes through the points [latex]\left(-2,6\right)[/latex] and [latex]\left(1,4\right)[/latex].

To write the equation of a line, the line’s slope and a point the line goes through must be known. Perhaps the most familiar form of a linear equation is slope-intercept form written as [latex]y=mx+b[/latex], where [latex]m=\text{slope}[/latex] and [latex]b=y\text{-intercept}[/latex]. Let us begin with the slope.

Often, the starting point to writing the equation of a line is to use point-slope formula. Given the slope and one point on a line, we can find the equation of the line using point-slope form shown below.

[latex]y-{y}_{1}=m\left(x-{x}_{1}\right)[/latex]

We need only one point and the slope of the line to use the formula. After substituting the slope and the coordinates of one point into the formula, we simplify it and write it in slope-intercept form.

A General Note: The Point-Slope Formula

Given one point and the slope, using point-slope form will lead to the equation of a line:

[latex]y-{y}_{1}=m\left(x-{x}_{1}\right)[/latex]

Example: Finding the Equation of a Line Given the Slope and One Point

Write the equation of the line with slope [latex]m=-3[/latex] and passing through the point [latex]\left(4,8\right)[/latex]. Write the final equation in slope-intercept form.

Try It

Given [latex]m=4[/latex], find the equation of the line in slope-intercept form passing through the point [latex]\left(2,5\right)[/latex].

Try It