Finding the derivative using the definition

In summary, the conversation discusses how to show that the function f(x)=x^(1/3) is not differentiable at zero. The conversation goes into using the definition of derivative and the Archimedian property to explain why the derivative is undefined at zero. It is mentioned that taking the limit in 0 simplifies the process.
  • #1
Punkyc7
420
0
show that f(x)= x^(1/3) is not differentiable at zero.
I know there is a vertical tangent at the origin and therefor not differentiable, but I'm not sure how to say it in analysis.

I have tried using caratheodry but it doesn't seem to get me anywhere
 
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  • #2
Can you find the derivative of f in 0?? Perhaps by applying the definition of derivative:

[tex]\lim_{h\rightarrow 0}\frac{f(h)-f(0)}{h}[/tex]

What is that derivative? Does it exist?
 
  • #3
No , I guess I should have said show show not differentiable using the definition.

Anyways

if I take the derivative I get

1/3 x^(-2/3)if x goes to 0 is undefined... But how do you say that in math talk
 
  • #4
You just say that the derivative is undefined. If you calculate the limit from the definition, then you say that the limit does not exist (because...?). When you're arguing like in your last post, you just say that it is undefined in 0 as division by 0 is undefined.
 
  • #5
does it violate the Archimedian property? I'm not sure how to use the definition because how do you expand (x+h)^(1/3)-(x)^1/3. I can't use the binomial theorem to expand it out because that only works for whole numbers.
 
  • #6
Punkyc7 said:
does it violate the Archimedian property? I'm not sure how to use the definition because how do you expand (x+h)^(1/3)-(x)^1/3. I can't use the binomial theorem to expand it out because that only works for whole numbers.

Do note that you take the limit in 0, so you can take x=0. This simplifies a whole lot.
 

1. What is the definition of a derivative?

The derivative of a function is the instantaneous rate of change of that function at a specific point. It represents the slope of the tangent line to the curve at that point.

2. How do you find the derivative using the definition?

To find the derivative using the definition, you must first write out the limit expression for the derivative, which is (f(x+h) - f(x)) / h. Then, substitute the given function for f(x) and simplify the expression to find the derivative.

3. Can you provide an example of finding the derivative using the definition?

For example, if we have the function f(x) = x^2, the derivative would be (f(x+h) - f(x)) / h = ((x+h)^2 - x^2) / h = (2xh + h^2) / h = 2x + h. As h approaches 0, the derivative becomes 2x, which represents the slope of the tangent line at any point on the curve.

4. Why is finding the derivative using the definition important?

Finding the derivative using the definition is important because it allows us to calculate the slope of a curve at any point, which is useful in many real-world applications such as physics, engineering, and economics. It also helps us understand the behavior of a function and its rate of change.

5. Are there other methods for finding the derivative?

Yes, there are other methods for finding the derivative, such as using the power rule, product rule, quotient rule, and chain rule. These methods are often quicker and more efficient than using the definition, but they all rely on the same fundamental concept of finding the instantaneous rate of change of a function at a specific point.

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