# How to Know if a Function is Differentiable – A Simple Guide

To determine if a function is differentiable, I first verify its continuity across its entire domain.

A function f(x) is considered differentiable at a point if it has a defined derivative at that point, meaning the slope of the tangent to the curve at that point exists.

I check this by calculating the derivative f'(x), and if it exists for all x within the domain of the function, then the function is differentiable on that domain.

In calculus, differentiability is crucial because it allows for the application of derivative-based techniques and fundamental theorems of calculus which are essential for analyzing the behavior of functions.

Understanding the relationship between continuity and differentiability is a key step. Although every differentiable function is continuous, the reverse isn’t always true. For a function to be differentiable at a point, it not only needs to be continuous there but also smooth, without any sharp corners or cusps.

As I explore further, I’ll remember that differentiability ensures a smooth, predictable change – a concept at the heart of calculus, enabling a deeper comprehension of change and motion.

Stay tuned; I’m about to uncover the world where the elegant dance of curves and slopes comes to life!

## Analyzing The Function’s Graph For Knowing Differentiability

When I look at a function’s graph, I’m seeking to determine if it is differentiable. A differentiable function typically means that there is a tangent line at every point along the graph.

For a function to be differentiable at a point, the tangent must exist, and the function must be continuous at that point. Moreover, the derivative of the function at that point must exist.

I first check for any discontinuities. If the graph shows any breaks, holes, or jumps, the function is not continuous there, implying non-differentiability. Next, the graph should not have any sharp corners or cusps—points where the tangent line is undefined because the slope changes abruptly.

Another visual clue for non-differentiability includes a vertical tangent line, indicating an infinite slope that defies the existence of a real-valued derivative. The limit of the slope of the secant lines should approach the slope of the tangent line as the points get infinitesimally close. This is formalized as:

$$\text{If the limit } \lim_{{h \to 0}} \frac{f(c + h) – f(c)}{h} \text{ exists, the function is differentiable at point } c.$$

A classic example of non-differentiability is the absolute value function at x=0 due to a sharp corner.

In summary, a differentiable function will have a graph where a unique tangent line is definable at every point within its domain. If a function’s graph meets these criteria, it is safe to conclude that the function exhibits differentiability.

## Exploring Examples and Exceptions

When I consider if a function is differentiable, I first check if the derivative of the function exists.

With polynomials, it’s typically straightforward due to the power rule, which states that the derivative of $x^n$ is $nx^{n-1}$. So, if I have $f(x) = x^4 – 3x + 5$, the derivative $f'(x) = 4x^3 – 3$ exists everywhere, making the function differentiable throughout its domain.

However, I also watch out for exceptions. Consider the absolute value function $f(x) = |x|$, which isn’t differentiable at $x=0$.

The left-hand limit of the derivative as $x$ approaches zero from the left ($\lim_{x \to 0^-} \frac{|x|}{x}$) is $-1$, while the right-hand limit as $x$ approaches zero from the right ($\lim_{x \to 0^+} \frac{|x|}{x}$) is $1$. Since these two limits do not match, the derivative does not exist at $x=0$.

I also encounter piecewise-defined functions that require careful examination. For such functions, I check the derivatives of each piece separately and at the points where the pieces meet.

For example, given $f(x) = \begin{cases} x^2 + 2 & \text{for } x \leq 1 \ -2x + 5 & \text{for } x > 1 \end{cases}$, I must ensure the slopes of the two pieces are equal at $x=1$ for $f(x)$ to be differentiable at that point.

I sometimes come across functions with vertical tangents or where the derivative heads to infinity, which also indicates non-differentiability at those points. Lastly, there are some pathological cases like the Weierstrass function, which is continuous everywhere but differentiable nowhere, defying my intuition!

In summary, my journey through examples and exceptions sharpens my understanding of the concept of differentiability.

## Conclusion

In determining whether a function is differentiable, I always consider the core principle that a differentiable function must have a derivative at every point within its domain.

I remember that for piecewise-defined functions, continuity at the junction point (points where the piecewise segments meet) is essential — they need to have matching left-hand and right-hand limits for both the function itself and its first derivative.

When I come across a function like $f(x) = \left{ \begin{array}{lr} x^2 + 2 & : x \leq 1\ -2x + 5 & : x > 1 \end{array} \right.$, I check its differentiability at $x=1$ by comparing the limits of its derivatives from the left and the right.

Similarly, for polynomials or transcendental functions, I can affirm their differentiability by verifying the existence of their derivatives.

In essence, for a function to be considered differentiable, it must have a well-defined derivative across its entire domain.

The existence and continuity of the derivative are my guiding lights here. Armed with these concepts and by applying derivative rules, I can tackle the question of differentiability with confidence and precision.