Intuition Behind Limits

A limit describes the value a function approaches as its input gets closer and closer to a certain point. What does this truly mean in practice? We will build some intuition around this concept.

Imagine you are looking at the graph of a function, like the simple linear function $f(x) = 2x + 1$ we saw earlier. Think of the limit as asking: "If I trace the graph with my finger, getting extremely close to a specific input value (say, $x=3$), where does it look like my finger is heading on the vertical axis (the $f(x)$ value)?"

Let's try this with $f(x) = 2x + 1$ as $x$ approaches 3. We aren't asking what $f(3)$ is (though in this simple case, it's $2(3) + 1 = 7$). Instead, we're exploring the neighborhood around $x=3$.

Consider values of $x$ slightly less than 3:

• If $x = 2.9$, then $f(x) = 2(2.9) + 1 = 5.8 + 1 = 6.8$
• If $x = 2.99$, then $f(x) = 2(2.99) + 1 = 5.98 + 1 = 6.98$
• If $x = 2.999$, then $f(x) = 2(2.999) + 1 = 5.998 + 1 = 6.998$

Now, consider values of $x$ slightly greater than 3:

• If $x = 3.1$, then $f(x) = 2(3.1) + 1 = 6.2 + 1 = 7.2$
• If $x = 3.01$, then $f(x) = 2(3.01) + 1 = 6.02 + 1 = 7.02$
• If $x = 3.001$, then $f(x) = 2(3.001) + 1 = 6.002 + 1 = 7.002$

Notice a pattern? As $x$ gets closer and closer to 3, from either side, the output value $f(x)$ gets closer and closer to 7. This value, 7, is the limit of the function $f(x) = 2x + 1$ as $x$ approaches 3. We write this mathematically as:

$$\lim_{x \to 3} (2x + 1) = 7$$

The graph below helps visualize this. As we pick points on the line closer to the vertical line where $x=3$, the corresponding points on the function get closer to the height $y=7$.

As input values x get closer to 3 from both the left (like 2.9, 2.99) and the right (like 3.1, 3.01), the output values f(x) get closer to 7. The limit describes this target value, 7.

The important part of the definition was "approaching a value without necessarily reaching it". For many simple functions like $f(x) = 2x + 1$, the limit as $x$ approaches a point $a$ is just the function's value at that point, $f(a)$. However, the concept of a limit is more general. It allows us to analyze function behavior even at points where the function might be undefined (like having a hole in the graph, though we won't focus on those tricky cases here). The limit cares about the trend of the function's output as the input gets arbitrarily close to a specific value.

Why spend time on this idea of "approaching"? Because it forms the foundation for understanding derivatives, which measure instantaneous rates of change. We need the idea of getting infinitely close to a point to talk about the slope or change right at that point. We'll see this connection clearly in the next chapter.