---
title: "Secant Line — AP Calculus Definition & Exam Guide"
description: "A secant line crosses a curve at two points, and its slope is the average rate of change. It's the bridge to the derivative and the Mean Value Theorem."
canonical: "https://fiveable.me/ap-calc/key-terms/secant-line"
type: "key-term"
subject: "AP Calculus AB/BC"
unit: "Unit 5"
---

# Secant Line — AP Calculus Definition & Exam Guide

## Definition

A secant line is a straight line that passes through two points on a curve, and its slope equals the average rate of change of the function between those points, computed as (f(b) − f(a))/(b − a) on the interval [a, b].

## What It Is

A secant line is a line drawn through two points on a curve. Its [slope](/ap-calc/key-terms/slope "fv-autolink") is just rise over run between those two points, which in calculus language is the **average rate of change** of the function on that interval. The CED gives the formula directly (EK for [AP Calc](/ap-calc "fv-autolink") 2.1.A): the average rate of change of f on [a, b] is (f(b) − f(a))/(b − a), provided a ≠ b. That expression is called a **difference quotient**, and it's literally the slope of the secant line through (a, f(a)) and (b, f(b)).

Here's the big idea the secant line sets up. Slide the two points closer and closer together, and the secant line tilts toward the **tangent line** at a single point. In the [limit](/ap-calc/unit-10-infinite-sequences-and-series-bc-only/defining-convergent-divergent-infinite-series/study-guide/CIVFHStGQM90EJ4GtIDB "fv-autolink"), the secant slope becomes the derivative. That's why the definition of the derivative looks like a secant slope with a limit stapled on the front: f'(a) = lim as h → 0 of (f(a+h) − f(a))/h. The secant line isn't a side concept. It's the picture behind what a derivative actually *is*.

## Why It Matters

Secant lines live in **Topic 2.1 ([Unit 2](/ap-calc/unit-2 "fv-autolink"): Differentiation: Definition and Fundamental Properties)** and come back in **Topic 5.1 (Unit 5: Analytical Applications of Differentiation)**. They directly support three learning objectives. For AP Calc 2.1.A, you compute average [rates of change](/ap-calc/key-terms/rate-of-change "fv-autolink") using difference quotients, which is secant slope. For AP Calc 2.1.B, you represent the derivative as the limit of a difference quotient, which is the secant slope collapsing into a tangent slope. Then in AP Calc 5.1.A, the **Mean Value Theorem** says that if f is continuous on [a, b] and differentiable on (a, b), some point c in the open interval has f'(c) equal to the secant slope over [a, b]. Geometrically, MVT guarantees a tangent line somewhere that's parallel to the secant line. If you can't picture a secant line, both the definition of the derivative and MVT turn into formula soup.

## Connections

### [Tangent Line (Unit 2)](/ap-calc/key-terms/tangent-line)

The [tangent line](/ap-calc/key-terms/tangent-line "fv-autolink") is what the secant line becomes when its two points merge into one. Every form of the derivative definition is just a secant slope inside a limit, so the tangent is the secant's limiting case.

### [Average Rate of Change (Unit 2)](/ap-calc/key-terms/average-rate-of-change)

These are two views of the same number. [Average rate of change](/ap-calc/key-terms/average-rate-of-change "fv-autolink") is the algebra, (f(b) − f(a))/(b − a), and the secant line is the geometry. If a question gives you one, you can always translate to the other.

### [Instantaneous Rate of Change (Unit 2)](/ap-calc/key-terms/instantaneous-rate-of-change)

[Instantaneous rate of change](/ap-calc/key-terms/instantaneous-rate-of-change "fv-autolink") at x = a is the limit of secant slopes as the second point slides toward a. Questions that ask 'what happens as the two points get closer together' are testing exactly this transition.

### Mean Value Theorem (Unit 5)

MVT is a promise about secant lines. If f is continuous on the closed interval and differentiable on the open interval, some interior point c has a tangent line parallel to the secant line over [a, b]. Drawing the secant first makes the theorem obvious.

## On the AP Exam

Secant lines show up two main ways. First, as a straight computation. A question hands you a function like f(x) = x³ + 2x − 4 and an interval like [−1, 2], and you find the secant slope with (f(2) − f(−1))/(2 − (−1)). This also appears with table data, where you pick two rows and compute the slope between them since you can't differentiate a table. Second, conceptually. Multiple-choice stems ask what the slope of a secant line represents (average rate of change) or what happens to that slope as the two points approach each other (it approaches the derivative). On FRQs, secant slopes are the engine behind two classic moves. You estimate a derivative from a table using the closest available interval, and you justify Mean Value Theorem conclusions by stating continuity and differentiability, then setting f'(c) equal to the secant slope. Always include correct units when the context is a rate, like meters per second.

## Secant Line vs Tangent Line

A secant line hits the curve at two points and gives the average rate of change over an interval. A tangent line touches at one point and gives the instantaneous rate of change there. The exam loves blurring this line, literally, because the tangent is defined as the limit of secant lines. Quick check: two points and an interval means secant and average; one point and a derivative means tangent and instantaneous.

## Key Takeaways

- The slope of a secant line equals the average rate of change of the function between the two points it passes through, computed as (f(b) − f(a))/(b − a).
- As the two points on a secant line get closer together, the secant slope approaches the slope of the tangent line, which is the derivative.
- The difference quotient in the definition of the derivative is just a secant slope, so the derivative is the limit of secant slopes.
- The Mean Value Theorem guarantees a point c in (a, b) where the tangent line is parallel to the secant line over [a, b], as long as f is continuous on [a, b] and differentiable on (a, b).
- When you only have a table of values, secant slopes are how you estimate derivatives, since you can't take an actual limit from a table.
- If a rate problem has real-world context, attach units to the secant slope, like meters per second or gallons per hour.

## FAQs

### What is a secant line in calculus?

A secant line is a straight line passing through two points on a curve. Its slope equals the average rate of change of the function between those points, given by (f(b) − f(a))/(b − a) on the interval [a, b].

### Is a secant line the same as a tangent line?

No. A secant line crosses the curve at two points and measures average rate of change, while a tangent line touches at one point and measures instantaneous rate of change. The tangent is the limit of secant lines as the two points merge.

### How do I find the slope of a secant line?

Plug both endpoints into the function and use slope formula. For f(x) = x³ + 2x − 4 on [−1, 2], compute (f(2) − f(−1))/(2 − (−1)) = (8 − (−7))/3 = 5.

### What does the slope of a secant line represent?

It represents the average rate of change of the function over the interval between the two points. In context problems, that means average velocity, average growth rate, and so on, with units.

### What does the secant line have to do with the Mean Value Theorem?

MVT says that if f is continuous on [a, b] and differentiable on (a, b), there's at least one point c in (a, b) where f'(c) equals the secant slope over [a, b]. In other words, some tangent line is parallel to the secant line.

## Related Study Guides

- [5.1 Using the Mean Value Theorem](/ap-calc/unit-5/using-mean-value-theorem/study-guide/79sP2PXcyvRvBsjb3HRq)
- [Unit 1 Overview: Limits and Continuity](/ap-calc/unit-1/review/study-guide/Y3NmqZtnvfKAdL2lDnaI)

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