Implicit differentiation — AP Calculus BC Study Guide

For: AP Calculus BC candidates sitting AP Calculus BC.

Covers: Covers key AP exam-focused skills including implicit vs explicit functions, the core chain-rule based implicit differentiation technique, first and higher-order implicit derivatives, tangent and normal lines to implicit curves, and derivatives of inverse functions via implicit differentiation.

You should already know: Chain rule for differentiation of composite functions. Derivatives of all basic elementary functions. Point-slope form for linear equations.

A note on the practice questions: All worked questions in the "Practice Questions" section below are original problems written by us in the AP Calculus BC style for educational use. They are not reproductions of past College Board / Cambridge / IB papers and may differ in wording, numerical values, or context. Use them to practise the technique; cross-check with official mark schemes for grading conventions.

1. What Is Implicit differentiation?

An explicit function is written in the form $y=f(x)$, where $y$ is isolated on one side and expressed purely in terms of $x$. An implicit relation is any equation that relates $x$ and $y$ without isolating $y$, and many such relations cannot be easily solved for $y$ at all (e.g., $x^3{y}^2-2xy+\ln (y)=5$).

Implicit differentiation is the technique to find $\frac{dy}{dx}$ directly from the implicit relation, without solving for $y$ first. The method relies entirely on the chain rule: since $y$ is a function of $x$ even if we cannot write it explicitly, any term containing $y$ is a composite function, so its derivative requires an extra factor of $\frac{dy}{dx}$.

According to the AP Calculus CED, this topic is part of Unit 3, which accounts for 9-13% of the total AP Calculus BC exam score. Implicit differentiation appears in both multiple-choice (MCQ) and free-response (FRQ) sections, and is often combined with other topics like tangents, inverse functions, and related rates.

2. Core Technique: Finding $\frac{dy}{dx}$ for Implicit Relations

The core logic of implicit differentiation is simple: if two expressions are equal for all valid $x$, their derivatives with respect to $x$ are also equal. We therefore differentiate every term on both sides of the equation with respect to $x$, and apply the chain rule to any term that contains $y$. Because $y=y(x)$, the derivative of $f(y)$ with respect to $x$ is $f^{\prime }(y)\cdot \frac{dy}{dx}$.

The step-by-step process is:

1. Differentiate every term on both sides of the equation with respect to $x$
2. For any term containing $y$, multiply by $\frac{dy}{dx}$ (the chain rule step)
3. Collect all terms with $\frac{dy}{dx}$ on one side of the equation, and all other terms on the opposite side
4. Factor out $\frac{dy}{dx}$, then divide both sides by the remaining coefficient to solve for $\frac{dy}{dx}$

Worked Example

Find $\frac{dy}{dx}$ for the implicit relation $x^2+3xy+y^2=5$.

1. Differentiate each term with respect to $x$: $$\frac{d}{dx}\left[x^2\right]+\frac{d}{dx}\left[3xy\right]+\frac{d}{dx}\left[y^2\right]=\frac{d}{dx}\left[5\right]$$
2. Compute derivatives, applying product rule to $3xy$ and chain rule to $y^2$: $$2x+3\left(y+x\frac{dy}{dx}\right)+2y\frac{dy}{dx}=0$$
3. Expand and collect $\frac{dy}{dx}$ terms on the left: $$2x+3y+\frac{dy}{dx}\left(3x+2y\right)=0⟹\frac{dy}{dx}\left(3x+2y\right)=-2x-3y$$
4. Solve for $\frac{dy}{dx}$: $$\frac{dy}{dx}=\frac{-2x-3y}{3x+2y}$$

Exam tip: Always simplify the final expression by factoring out common constants or negative signs before moving on; most MCQ answer choices use the simplified form, and an extra common factor will cost you a point.

3. Higher-Order Derivatives Implicitly

Once you find $\frac{dy}{dx}$ in terms of $x$ and $y$, you can find higher-order derivatives (most commonly the second derivative $\frac{d^{2}y}{d{x}^2}$) by differentiating $\frac{dy}{dx}$ again with respect to $x$. The key rule here is that any time you differentiate a term containing $y$, you still need to apply the chain rule and multiply by $\frac{dy}{dx}$.

After differentiating, you must substitute the expression you already found for $\frac{dy}{dx}$ into the result to get the final second derivative in terms of only $x$ and $y$, with no remaining $\frac{dy}{dx}$ terms. Often, the original implicit relation can be used to simplify the final result significantly, which is a common AP exam trick.

Worked Example

Given $x^2+3xy+y^2=5$ and $\frac{dy}{dx}=\frac{-2x-3y}{3x+2y}$, find $\frac{d^{2}y}{d{x}^2}$.

1. Differentiate $\frac{dy}{dx}$ using the quotient rule: $$\frac{d^{2}y}{d{x}^2}=\frac{\left(-2-3\frac{dy}{dx}\right)(3x+2y)-(-2x-3y)\left(3+2\frac{dy}{dx}\right)}{(3x+2y{)}^2}$$
2. Expand and simplify the numerator: $$-6x-4y-9x\frac{dy}{dx}-6y\frac{dy}{dx}+6x+4x\frac{dy}{dx}+9y+6y\frac{dy}{dx}=5y-5x\frac{dy}{dx}$$
3. Substitute $\frac{dy}{dx}=\frac{-2x-3y}{3x+2y}$ into the numerator: $$5y-5x\left(\frac{-2x-3y}{3x+2y}\right)=\frac{5y(3x+2y)+5x(2x+3y)}{3x+2y}=\frac{10(x^2+3xy+y^2)}{3x+2y}$$
4. Use the original relation $x^2+3xy+y^2=5$ to simplify: $$\frac{d^{2}y}{d{x}^2}=\frac{10(5)}{(3x+2y{)}^3}=\frac{50}{(3x+2y{)}^3}$$

Exam tip: Always check if the original implicit relation can be substituted into the second derivative to simplify the numerator; this cuts your work in half and avoids unnecessary algebraic errors.

4. Tangent and Normal Lines to Implicit Curves

One of the most common AP exam applications of implicit differentiation is finding the equation of a tangent line or normal line to an implicit curve at a given point. By definition, $\frac{dy}{dx}$ at a point $(x_0,y_0)$ on the curve is exactly the slope of the tangent line at that point. The normal line is perpendicular to the tangent line, so its slope is the negative reciprocal of the tangent slope, as long as the tangent slope is defined and non-zero.

If $\frac{dy}{dx}=0$ at the point, the tangent line is horizontal and the normal line is vertical. If $\frac{dy}{dx}$ is undefined at the point, the tangent line is vertical and the normal line is horizontal.

Worked Example

Find the equation of the tangent line to the curve $x^2+3xy+y^2=5$ at the point $(1,1)$.

1. First confirm that $(1,1)$ lies on the curve: $1^2+3(1)(1)+1^2=5$, which matches the right-hand side.
2. We already found $\frac{dy}{dx}=\frac{-2x-3y}{3x+2y}$. Evaluate at $(1,1)$: $$\frac{dy}{dx}{|}_{(1,1)}=\frac{-2(1)-3(1)}{3(1)+2(1)}=\frac{-5}{5}=-1$$
3. Use point-slope form to write the tangent line equation: $$y-1=-1(x-1)$$
4. Simplify to slope-intercept form: $y=-x+2$

Exam tip: Always confirm the given point lies on the curve before calculating slope; AP problems sometimes include an implicit check step, and off-curve points will give incorrect tangent lines.

5. Derivatives of Inverse Functions via Implicit Differentiation

Implicit differentiation is the simplest way to derive the derivative of any inverse function, including inverse trigonometric functions. By definition, if $y=f^{-1}(x)$, then $f(y)=x$, which is an implicit relation. We can differentiate both sides with respect to $x$ and solve for $\frac{dy}{dx}$ directly, which gives the inverse derivative formula without memorization.

Worked Example

Find the derivative of $y=\arctan (x)$ using implicit differentiation.

1. By definition of the arctangent inverse function, $\tan (y)=x$, where $-\frac{\pi }{2}<y<\frac{\pi }{2}$.
2. Differentiate both sides with respect to $x$, applying the chain rule to $\tan (y)$: $${\sec }^2(y)\cdot \frac{dy}{dx}=1$$
3. Solve for $\frac{dy}{dx}$: $$\frac{dy}{dx}=\frac{1}{{\sec }^2(y)}$$
4. Use the Pythagorean identity ${\sec }^2(y)=1+{\tan }^2(y)$, and substitute $\tan (y)=x$: $$\frac{dy}{dx}=\frac{1}{1+{\tan }^2(y)}=\frac{1}{1+x^2}$$

Exam tip: Always recall the principal range of the inverse function when simplifying; this determines the sign of any root you take, and a wrong sign gives the wrong derivative.

6. Common Pitfalls (and how to avoid them)

• Wrong move: Forgetting to multiply the derivative of a $y$-term by $\frac{dy}{dx}$, e.g., writing $\frac{d}{dx}[y^2]=2y$ instead of $2y\frac{dy}{dx}$. Why: Students get used to differentiating functions of $x$ and forget $y$ depends on $x$, so the chain rule requires the extra factor. Correct move: Every time you differentiate a term that contains a $y$, immediately write $\frac{dy}{dx}$ after it before moving to the next term.
• Wrong move: Leaving $\frac{dy}{dx}$ in the final expression for $\frac{d^{2}y}{d{x}^2}$ without substituting. Why: Students forget the second derivative must be in terms of $x$ and $y$, not $\frac{dy}{dx}$. Correct move: After differentiating $\frac{dy}{dx}$, immediately substitute the expression you already found for $\frac{dy}{dx}$ before simplifying.
• Wrong move: When applying product rule to $xy$, writing $\frac{d}{dx}[xy]=x\frac{dy}{dx}+y\frac{dy}{dx}$. Why: Students incorrectly add $\frac{dy}{dx}$ to both factors, confusing which factor depends on $y$. Correct move: Apply the product rule first, then only add $\frac{dy}{dx}$ when differentiating the factor that contains $y$.
• Wrong move: Calculating the tangent line equation using the general form of $\frac{dy}{dx}$ instead of the evaluated slope. Why: Students rush to write the line before getting a numerical slope at the given point. Correct move: Evaluate $\frac{dy}{dx}$ at the given point to get a numerical slope before writing the line equation.
• Wrong move: Solving for $y$ first when an implicit relation can be split into two explicit branches, leading to only one valid derivative. Why: Students default to explicit differentiation even when splitting loses information. Correct move: Always use implicit differentiation for relations, it automatically works for all branches.

7. Practice Questions (AP Calculus BC Style)

Question 1 (Multiple Choice)

Which of the following is equal to $\frac{dy}{dx}$ for the implicit relation $2x^{3}y+\ln (y)=4$? A) $\frac{-6x^2{y}^2}{2x^{3}y+1}$ B) $\frac{-6x^{2}y}{2x^3+\frac{1}{y}}$ C) $\frac{6x^{2}y}{2x^3+\frac{1}{y}}$ D) $\frac{-2x^{3}y}{6x^{2}y+1}$

Worked Solution: Differentiate both sides of $2x^{3}y+\ln (y)=4$ with respect to $x$, using product rule for $2x^{3}y$ and chain rule for $\ln (y)$. This gives $6x^{2}y+2x^3\frac{dy}{dx}+\frac{1}{y}\frac{dy}{dx}=0$. Collect terms with $\frac{dy}{dx}$ on the left side: $\frac{dy}{dx}\left(2x^3+\frac{1}{y}\right)=-6x^{2}y$. Divide both sides by $2x^3+\frac{1}{y}$ to get $\frac{dy}{dx}=\frac{-6x^{2}y}{2x^3+\frac{1}{y}}$. The correct answer is B.

Question 2 (Free Response)

Consider the curve defined by $x^3-xy+y^2=7$. (a) Find $\frac{dy}{dx}$ in terms of $x$ and $y$. (b) Find the equation of the normal line to the curve at the point $(2,1)$. (c) Find $\frac{d^{2}y}{d{x}^2}$ at the point $(2,1)$.

Worked Solution: (a) Differentiate term-by-term with respect to $x$: $$3x^2-\left(y+x\frac{dy}{dx}\right)+2y\frac{dy}{dx}=0$$ Rearrange and solve for $\frac{dy}{dx}$: $$\frac{dy}{dx}(-x+2y)=y-3x^2⟹\frac{dy}{dx}=\frac{y-3x^2}{2y-x}$$

(b) Confirm $(2,1)$ is on the curve: $8-(2)(1)+1=7$, which matches. Evaluate $\frac{dy}{dx}$ at $(2,1)$: $\frac{1-12}{2(1)-2}=\frac{-11}{0}$, so $\frac{dy}{dx}$ is undefined. This means the tangent line is vertical ($x=2$), so the normal line (perpendicular to vertical) is horizontal with slope $0$. The equation is $y=1$.

(c) $\frac{dy}{dx}$ is undefined at $(2,1)$, so $\frac{d^{2}y}{d{x}^2}$ is also undefined at this point, as the first derivative does not exist here.

Question 3 (Application / Real-World Style)

A lens manufacturer designs a convex lens with a cross-section (upper half) defined by the implicit relation $9x^2+16y^2-2x^3=112$, where $x$ and $y$ are measured in centimeters, and $y>0$. The slope of the tangent line at a point on the lens edge determines the angle of refraction of incoming light. Find the slope of the upper edge of the lens at $x=2$, and interpret the result.

Worked Solution: First find $y$ when $x=2$: $9(4)+16y^2-2(8)=36+16y^2-16=20+16y^2=112⟹16y^2=92⟹y=\frac{\sqrt{23}}{2}$ (take positive $y$ for upper half). Differentiate the relation with respect to $x$: $18x+32y\frac{dy}{dx}-6x^2=0$. Solve for $\frac{dy}{dx}$: $\frac{dy}{dx}=\frac{6x^2-18x}{32y}=\frac{3x^2-9x}{16y}$. Evaluate at $x=2$: $\frac{dy}{dx}=\frac{12-18}{16(\sqrt{23}/2)}=\frac{-6}{8\sqrt{23}}\approx -0.078$.

Interpretation: At 2 centimeters from the center of the lens, the upper edge decreases by approximately 0.078 centimeters for every 1 centimeter increase in horizontal distance from the center, giving a shallow downward slope on the right side of the lens.

8. Quick Reference Cheatsheet

9. What's Next

Implicit differentiation is the foundational technique for all remaining topics in this unit, and it enables key applications across the rest of the AP Calculus BC course. Next you will apply implicit differentiation to logarithmic differentiation, which simplifies differentiation of complicated products, quotients, and variable exponents that cannot be differentiated with basic rules. Without mastering implicit differentiation, you will not be able to correctly derive derivatives of inverse functions, solve related rates problems, or find tangents to parametric and polar curves later in the course. Beyond this unit, implicit differentiation is required for all implicit optimization problems and BC-specific topics like slope fields for implicit relations.

Logarithmic differentiation Derivatives of inverse functions Related rates Tangents to parametric curves