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Understanding Limits Intuitively

Reference: Stewart 2.2 • Chapter: 1 • Section: 2

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What Does "Approaching" Really Mean?

Imagine you're walking toward a door. You take a step, then half a step, then a quarter step... You're getting arbitrarily close to the door without actually touching it. This is exactly what a limit captures: the behavior of a function as its input approaches a value, regardless of what happens at that value.

The limit is one of the most powerful ideas in calculus because it lets us precisely describe "trends" and "tendencies." When we write $\lim_{x \to a} f(x) = L$, we're saying: "As $x$ gets closer and closer to $a$ (but doesn't equal $a$), $f(x)$ gets closer and closer to $L$."

The key insight: The limit depends on approach, not arrival. The function might not even be defined at $a$, and that's perfectly fine!

Prerequisite Map

Quick Reference

Property	Value
Concept	Limits
Course	MATH161
Section	Stewart 1.5
Difficulty	Beginner
Time	~15 minutes

Key Concepts

The Limit Definition (Intuitive)

$$\lim_{x \to a} f(x) = L$$

This notation means: As $x$ gets arbitrarily close to $a$ (but $x \neq a$), $f(x)$ gets arbitrarily close to $L$.

An equivalent notation is: $f(x) \to L$ as $x \to a$.

Critical distinction: The limit asks "What value is $f(x)$ approaching?" not "What is $f(a)$?"

The Limit vs. The Function Value

Scenario	$f(a)$	$\lim_{x \to a} f(x)$	What's Happening
Continuous at $a$	Defined, equals $L$	Equals $L$	Everything agrees
Hole at $a$	Undefined	Still can be $L$	Limit exists despite the hole
Jump at $a$	Defined	Does not exist	Left and right limits differ
Asymptote at $a$	Undefined	$\pm\infty$ or DNE	Function blows up

Visualizing the Limit

Consider $f(x) = \frac{x^2 - 1}{x - 1}$ near $x = 1$:

    y
    |
  3 +
    |
  2 +--------○--------  ← hole at (1, 2)
    |      /
  1 +    /
    |  /
  0 +--------+--------→ x
    0        1        2

Even though $f(1)$ is undefined (division by zero), the limit as $x \to 1$ is $2$ because that's where $f(x)$ is heading.

Why The Value at $a$ Doesn't Matter

The limit only cares about values of $x$ near $a$, not at $a$. This is what makes limits so powerful:

We can study behavior at points where the function isn't defined
We can define instantaneous velocity (derivative) as a limit
We can define the area under a curve (integral) as a limit

Memorable rule: The limit is about the journey, not the destination.

Common Notation

Notation	Meaning
$\lim_{x \to a} f(x) = L$	The limit of $f(x)$ as $x$ approaches $a$ is $L$
$f(x) \to L$ as $x \to a$	Same meaning, alternative notation
DNE	"Does not exist": limit fails to exist

Practice Problems

Level 1 Interpreting Limit Notation

What does the statement $\lim_{x \to 3} f(x) = 7$ mean in plain English?

Thought Process

The limit notation has three parts: the input value being approached ($3$), the function ($f$), and the output being approached ($7$). Translate each part into everyday language.

Show Answer

As $x$ gets closer and closer to $3$ (but doesn't equal $3$), the values of $f(x)$ get closer and closer to $7$.

Note: This says nothing about what $f(3)$ equals: it might be $7$, might be something else, or might not exist at all.

Level 2 Limit from a Table

Given the following table of values for $g(x)$, estimate $\lim_{x \to 2} g(x)$.

$x$	1.9	1.99	1.999	2.001	2.01	2.1
$g(x)$	4.7	4.97	4.997	5.003	5.03	5.3

Thought Process

Look at what happens to $g(x)$ as $x$ approaches $2$ from both sides. From the left (1.9, 1.99, 1.999), $g(x)$ approaches... what value? From the right (2.001, 2.01, 2.1), $g(x)$ approaches... the same value?

Show Answer

From the left: $4.7 \to 4.97 \to 4.997 \to \ldots$ approaching $5$

From the right: $5.003 \to 5.03 \to 5.3 \to \ldots$ also approaching $5$

Since both sides approach the same value:

$$\lim_{x \to 2} g(x) = 5$$

Level 3 Limit vs. Function Value

Let $f(x) = \begin{cases} x + 1 & \text{if } x \neq 2 \\ 10 & \text{if } x = 2 \end{cases}$

What is $f(2)$?
What is $\lim_{x \to 2} f(x)$?
Are they equal? What does this tell you about continuity?

Thought Process

For part (a), use the piecewise definition directly at $x = 2$. For part (b), remember the limit only cares about values near 2, not at 2, so which piece of the function applies? For part (c), compare the two answers.

Show Answer

(a) $f(2) = 10$ (by the second piece of the definition)

(b) For $x \neq 2$, we have $f(x) = x + 1$. As $x \to 2$: $$\lim_{x \to 2} f(x) = \lim_{x \to 2} (x + 1) = 2 + 1 = 3$$

(c) No, they are not equal: $f(2) = 10 \neq 3 = \lim_{x \to 2} f(x)$

This tells us $f$ is not continuous at $x = 2$. The function has a "jump" discontinuity: there's a hole in the line $y = x + 1$ at $x = 2$, with a separate point at $(2, 10)$.

Level 4 Graphical Limit Analysis

The graph of $h(x)$ is shown below:

    y
    |
  4 +                    ●
    |
  3 +            ○------/
    |           /
  2 +       ---/
    |      /
  1 +    ●
    |
  0 +----+----+----+----+→ x
        1    2    3    4

Key: ● = filled point (function value), ○ = open circle (hole)

Find each of the following, or state that it does not exist:

$\lim_{x \to 2} h(x)$
$h(2)$
$\lim_{x \to 3} h(x)$
$h(3)$

Thought Process

For limits, trace along the curve and see where you're heading (ignore holes and filled points; they don't affect the limit). For function values, look at where the filled point is (or note if there's only a hole).

Show Answer

(a) $\lim_{x \to 2} h(x) = 2$ Following the curve from either side, we approach the point at height $2$.

(b) $h(2) = 2$ The filled point at $x = 2$ is at height $2$ (on the curve).

(c) $\lim_{x \to 3} h(x) = 3$ Following the curve, we approach the open circle at height $3$.

(d) $h(3) = 4$ The filled point at $x = 3$ is at height $4$ (separate from the curve).

Note: At $x = 2$, the limit and function value agree (continuous there). At $x = 3$, they differ (discontinuous there).

Level 5 Constructing a Function with Specified Limit Behavior

Construct a piecewise function $f(x)$ that satisfies ALL of the following conditions:

$\lim_{x \to 1} f(x) = 4$
$f(1) = 7$
$f$ is defined for all real numbers
$f(x) = x^2 + 3$ for all $x > 1$

Then explain why your function satisfies each condition.

Thought Process

Work backwards from the conditions:

Condition (d) tells us what $f$ looks like for $x > 1$
From the right, as $x \to 1^+$, we get $f(x) \to 1^2 + 3 = 4$ ✓ (matches condition a)
We need the left-hand limit to also equal $4$
At $x = 1$ itself, we need $f(1) = 7$ (not $4$!)

So we need: a formula for $x < 1$ that approaches $4$ as $x \to 1^-$, a special value at $x = 1$, and the given formula for $x > 1$.

Show Answer

One valid answer:

$$f(x) = \begin{cases} x + 3 & \text{if } x < 1 \\ 7 & \text{if } x = 1 \\ x^2 + 3 & \text{if } x > 1 \end{cases}$$

Verification:

(a) From the left: $\lim_{x \to 1^-} (x + 3) = 1 + 3 = 4$

From the right: $\lim_{x \to 1^+} (x^2 + 3) = 1 + 3 = 4$

Both one-sided limits equal $4$, so $\lim_{x \to 1} f(x) = 4$ ✓

(b) By definition, $f(1) = 7$ ✓

(c) The function is defined by one of the three cases for any real $x$ ✓

(d) For $x > 1$, $f(x) = x^2 + 3$ by the third case ✓

Note: Many other formulas work for $x < 1$ (e.g., $5 - x$, $2x + 2$, $4$, etc.) as long as the left-hand limit equals $4$.

Mastery Checklist

[ ] I can explain in plain language what $\lim_{x \to a} f(x) = L$ means
[ ] I understand why the limit can exist even when $f(a)$ is undefined
[ ] I can distinguish between the limit and the function value at a point
[ ] I can estimate limits from tables of values
[ ] I can read limits from graphs, identifying where holes and filled points are
[ ] I can construct piecewise functions with specified limit behavior

Mental Model

The Spotlight Analogy: Think of a spotlight that can shine anywhere except at the center. The limit asks: "What value is illuminated as the spotlight gets infinitely close to the center?" The center itself is in darkness; we never look there. This is why $f(a)$ doesn't matter; we're only studying the lit region around $a$.

Connections

Looking back:

Function evaluation lets us compute $f(x)$ at specific points; limits extend this to describe behavior near points
Graphing skills help visualize the "approaching" behavior

Looking ahead:

One-Sided Limits handle cases where behavior differs from left and right
Continuity is defined in terms of limits
The derivative is defined as a limit: $f'(a) = \lim_{h \to 0} \frac{f(a+h) - f(a)}{h}$

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Ch1 §4	Skills Index	One-Sided Limits

Last updated: 2026-01-22