Derivatives of absolute values

In summary, the function f(x)=|x| is differentiable for all values of x except for 0. This is because at x=0, the graph of f(x) has a corner or cusp, which means the slope of the tangent line is undefined. However, for all other values of x, the function is differentiable and can be rewritten as a piecewise function with different derivatives for x>0 and x<0.
  • #1
fk378
367
0

Homework Statement


Where is the function f(x) = |x| differentiable?


Homework Equations


[f(x+h) - f(x)] / h


The Attempt at a Solution


I know that the graph of f(x)=|x| shows a corner at the origin from which 2 lines project at opposite slopes, as in they are symmetric about the y-axis.
I've seen the solution but I don't understand why f(x) is differentiable for every number except 0.
 
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  • #2
|x|=x for x>0 and |x|=(-x) for x<0. x and -x are both differentiable. Remember the derivative expression is a limit. It's only correct in the limit where h->0. Think of h as REALLY SMALL.
 
  • #3
You can re-write |x| as a piecewise function

f(x) = -x, x<0
f(x) = 0, x=0
f(x) = x, x>0

Find the derivative and I think you'll see your answer -- Oh and you might want to read up on what a "cusp" is.
 

Related to Derivatives of absolute values

1. What is the derivative of an absolute value function?

The derivative of an absolute value function is a piecewise function, where the derivative is -1 if the input is negative, 1 if the input is positive, and undefined at the point where the input is 0.

2. How do you find the derivative of a piecewise function with absolute value?

To find the derivative of a piecewise function with absolute value, you need to break the function into two separate cases: one for when the input is positive and one for when the input is negative. Then, you can use the power rule and chain rule to find the derivative for each case.

3. Can the derivative of an absolute value function be negative?

No, the derivative of an absolute value function is always either 1 or -1. This is because the derivative of an absolute value function is a piecewise function, and the derivative is defined as -1 for negative inputs and 1 for positive inputs.

4. How can you use the derivative of an absolute value function to find critical points?

The derivative of an absolute value function can help identify critical points, which are points where the slope of the function is 0. At these points, the derivative of the absolute value function is undefined. So, to find critical points, you can set the derivative equal to 0 and solve for the input value.

5. Can the derivative of an absolute value function have a point of discontinuity?

Yes, the derivative of an absolute value function can have a point of discontinuity. This occurs at the point where the input is 0, as the derivative is undefined at this point. This is because the derivative of an absolute value function is a piecewise function and is not continuous at the point where the input is 0.

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