What is a Limit? (The Big Picture)

Welcome to Calculus!

Let's start with the most important concept: the limit

Before we dive into formulas, let's understand what calculus is actually about. Calculus answers one fundamental question: How do things change?

Think about it - the world is constantly changing. Cars accelerate, populations grow, temperatures fluctuate. Calculus gives us the tools to analyze and predict these changes.

And it all starts with a simple but powerful idea: the limit.

💡

The Walking-to-a-Wall Analogy

Imagine you're 10 meters from a wall. You walk halfway there (5m remaining). Then halfway again (2.5m). Then halfway again (1.25m).

You keep going: 0.625m... 0.3125m... 0.156m...

Question: Will you ever actually reach the wall?

Mathematically: No! You're always covering half the remaining distance, so there's always some distance left.

But here's the key insight: You're approaching the wall. The distance is getting closer and closer to zero. In calculus, we'd say:

The limit of your distance from the wall is zero.

The limit tells us what value you're approaching, even if you never quite get there.

Formal Definition

Limit

lim[x->a] f(x) = L

"The limit of f(x) as x approaches a equals L"

As the input x gets closer and closer to the value 'a', the output f(x) gets closer and closer to the value 'L'.

x:: The input variable (independent variable)
a:: The value x is approaching (but may never reach)
f(x):: The function - the rule that transforms x into an output
L:: The limit - the value the output is approaching

⚠️

Critical Understanding

The limit is about approaching, not arriving. The function doesn't need to actually equal L when x = a. In fact, the function might not even be defined at x = a! The limit only cares about what happens as you get close to a.

Your First Limit

Easy

Find lim[x->3] (x^2)

1

Understand the question

We want to know: as x gets closer to 3, what does x^2 get closer to?

2

Test some values approaching 3

Let's try values getting closer to 3 from the left: - x = 2.9 -> x^2 = 8.41 - x = 2.99 -> x^2 = 8.94 - x = 2.999 -> x^2 = 8.994 And from the right: - x = 3.1 -> x^2 = 9.61 - x = 3.01 -> x^2 = 9.06 - x = 3.001 -> x^2 = 9.006

3

Identify the pattern

From both sides, x^2 is getting closer and closer to **9**.

4

Write the answer

lim[x->3] (x^2) = 9

Answer: 9

💡 For this simple function, the limit equals the value you get by plugging in x = 3. That's called **direct substitution** - and it works for most 'nice' functions. But as we'll see, not all functions are this nice!

Quick Check

Make sure you've got the basic idea before moving on.

1. What does lim[x->5] f(x) = 10 mean?

f(5) equals 10

As x gets close to 5, f(x) gets close to 10

x can never equal 5

f(x) can never equal 10

The limit tells us what value f(x) is APPROACHING as x approaches 5. It doesn't tell us anything about what actually happens AT x = 5.

2. Find lim[x->2] (3x + 1)

5

6

7

8

As x approaches 2, 3x + 1 approaches 3(2) + 1 = 7. For this simple function, we can just substitute!

Key Takeaways

A limit describes what value a function is approaching
The limit doesn't care what happens AT the point - only what happens NEAR it
Notation: lim[x->a] f(x) = L means 'f(x) approaches L as x approaches a'
For simple functions, you can often find limits by direct substitution