3. Derivatives

# 3.8 Implicit Differentiation

### Learning Objectives

- Find the derivative of a complicated function by using implicit differentiation.
- Use implicit differentiation to determine the equation of a tangent line.

We have already studied how to find equations of tangent lines to functions and the rate of change of a function at a specific point. In all these cases we had the explicit equation for the function and differentiated these functions explicitly. Suppose instead that we want to determine the equation of a tangent line to an arbitrary curve or the rate of change of an arbitrary curve at a point. In this section, we solve these problems by finding the derivatives of functions that define implicitly in terms of .

# Implicit Differentiation

In most discussions of math, if the dependent variable is a function of the independent variable , we express in terms of . If this is the case, we say that is an *explicit function* of . For example, when we write the equation , we are defining *explicitly* in terms of . On the other hand, if the relationship between the function and the variable is expressed by an equation where is not expressed entirely in terms of , we say that the equation defines *implicitly* in terms of . For example, the equation defines the function implicitly.

**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 are functions that satisfy the given equation, but that is not actually a function of .

In general, an equation defines a function implicitly if the function satisfies that equation. An equation may define many different functions implicitly. For example, the functions

, , and , which are illustrated in (Figure), are just three of the many functions defined implicitly by the equation .

If we want to find the slope of the line tangent to the graph of at the point , we could evaluate the derivative of the function at . On the other hand, if we want the slope of the tangent line at the point , we could use the derivative of . However, it is not always easy to solve for a function defined implicitly by an equation. Fortunately, the technique of implicit differentiation allows us to find the derivative of an implicitly defined function without ever solving for the function explicitly. The process of finding using implicit differentiation is described in the following problem-solving strategy.

### Problem-Solving Strategy: Implicit Differentiation

To perform implicit differentiation on an equation that defines a function implicitly in terms of a variable , use the following steps:

- Take the derivative of both sides of the equation. Keep in mind that is a function of . Consequently, whereas because we must use the Chain Rule to differentiate with respect to .
- Rewrite the equation so that all terms containing are on the left and all terms that do not contain are on the right.
- Factor out on the left.
- Solve for by dividing both sides of the equation by an appropriate algebraic expression.

### Using Implicit Differentiation

Assuming that is defined implicitly by the equation , find .

#### Solution

Follow the steps in the problem-solving strategy.

### Using Implicit Differentiation and the Product Rule

Assuming that is defined implicitly by the equation , find .

#### Solution

### Using Implicit Differentiation to Find a Second Derivative

Find if .

#### Solution

In (Figure), we showed that . We can take the derivative of both sides of this equation to find .

At this point we have found an expression for . If we choose, we can simplify the expression further by recalling that and making this substitution in the numerator to obtain .

Find for defined implicitly by the equation .

#### Solution

#### Hint

Follow the problem solving strategy, remembering to apply the chain rule to differentiate and .

# Finding Tangent Lines Implicitly

Now that we have seen the technique of implicit differentiation, we can apply it to the problem of finding equations of tangent lines to curves described by equations.

### Finding a Tangent Line to a Circle

Find the equation of the line tangent to the curve at the point .

#### Solution

Although we could find this equation without using implicit differentiation, using that method makes it much easier. In (Figure), we found .

The slope of the tangent line is found by substituting into this expression. Consequently, the slope of the tangent line is .

Using the point and the slope in the point-slope equation of the line, we then solve for to obtain the equation ((Figure)).

### Finding the Equation of the Tangent Line to a Curve

Find the equation of the line tangent to the graph of at the point ((Figure)). This curve is known as the folium (or leaf) of Descartes.

#### Solution

Begin by finding

Next, substitute into to find the slope of the tangent line:

Finally, substitute into the point-slope equation of the line and solve for to obtain

### Applying Implicit Differentiation

In a simple video game, a rocket travels in an elliptical orbit whose path is described by the equation . The rocket can fire missiles along lines tangent to its path. The object of the game is to destroy an incoming asteroid traveling along the positive -axis toward . If the rocket fires a missile when it is located at , where will it intersect the -axis?

#### Solution

To solve this problem, we must determine where the line tangent to the graph of

at intersects the -axis. Begin by finding implicitly.

Differentiating, we have

Solving for , we have

The slope of the tangent line is . The equation of the tangent line is . To determine where the line intersects the -axis, solve . The solution is . The missile intersects the -axis at the point .

Find the equation of the line tangent to the hyperbola at the point .

#### Solution

#### Hint

Using implicit differentiation, you should find that .

### Key Concepts

- We use implicit differentiation to find derivatives of implicitly defined functions (functions defined by equations).
- By using implicit differentiation, we can find the equation of a tangent line to the graph of a curve.

For the following exercises, use implicit differentiation to find .

**1. **

**2. **

#### Solution

**3. **

**4. **

#### Solution

**5. **

**6. **

#### Solution

**7. **

**8. **

#### Solution

**9. **

**10. **

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For the following exercises, find the equation of the tangent line to the graph of the given equation at the indicated point. Use a calculator or computer software to graph the function and the tangent line.

**11. [T] **

**12. [T] **

#### Solution

**13. [T] **

**14. [T] **

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**15. [T] **

**16. [T] **

#### Solution

**17. [T]** The graph of a folium of Descartes with equation is given in the following graph.

- Find the equation of the tangent line at the point . Graph the tangent line along with the folium.
- Find the equation of the normal line to the tangent line in a. at the point .

**18. **For the equation ,

- Find the equation of the normal to the tangent line at the point .
- At what other point does the normal line in a. intersect the graph of the equation?

#### Solution

a.

b.

**19. **Find all points on the graph of at which the tangent line is vertical.

**20. **For the equation ,

- Find the -intercept(s).
- Find the slope of the tangent line(s) at the -intercept(s).
- What does the value(s) in b. indicate about the tangent line(s)?

#### Solution

a.

b. Slope is -2 at both intercepts

c. They are parallel since the slope is the same at both intercepts.

**21. **Find the equation of the tangent line to the graph of the equation at the point .

**22. **Find the equation of the tangent line to the graph of the equation at the point .

#### Solution

**23. **Find and for .

**24. [T]** The number of cell phones produced when dollars is spent on labor and dollars is spent on capital invested by a manufacturer can be modeled by the equation .

- Find and evaluate at the point .
- Interpret the result of a.

#### Solution

a. -0.5926

b. When $81 is spent on labor and $16 is spent on capital, the amount spent on capital is decreasing by $0.5926 per $1 spent on labor.

**25. [T]** The number of cars produced when dollars is spent on labor and dollars is spent on capital invested by a manufacturer can be modeled by the equation .

(Both and are measured in thousands of dollars.)

- Find and evaluate at the point .
- Interpret the result of a.

**26. **The volume of a right circular cone of radius and height is given by . Suppose that the volume of the cone is . Find when and .

#### Solution

For the following exercises, consider a closed rectangular box with a square base with side and height .

**27. **Find an equation for the surface area of the rectangular box, .

**28. **If the surface area of the rectangular box is 78 square feet, find when feet and feet.

#### Solution

For the following exercises, use implicit differentiation to determine . Does the answer agree with the formulas we have previously determined?

**29. **

**30. **

#### Solution

**31. **

## Glossary

- implicit differentiation
- is a technique for computing for a function defined by an equation, accomplished by differentiating both sides of the equation (remembering to treat the variable as a function) and solving for

## Analysis

Note that the resulting expression for is in terms of both the independent variable and the dependent variable . Although in some cases it may be possible to express in terms of only, it is generally not possible to do so.