Derive x/(3x-1) using first principles

  • Thread starter Zipzap
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In summary, the conversation is about a person who is stuck on a problem involving the first principles formula for finding derivatives. They are unable to get two separate terms in their answer and are seeking help. Another person suggests using the chain rule and simplifying the expression, concluding that the original answer is actually correct.
  • #1
Zipzap
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Ok, I am really stuck on this one. I tried using the first principles formula and everything, but I don't get two separate terms like I am supposed to in the actual derivative. I always end up with -1/(3x-1)^2 when I try. Can someone please help me out?
 
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  • #2
Zipzap said:
Ok, I am really stuck on this one. I tried using the first principles formula and everything, but I don't get two separate terms like I am supposed to in the actual derivative. I always end up with -1/(3x-1)^2 when I try. Can someone please help me out?

Show us what you've done. It should start off something like this:
[tex]\lim_{h \to 0}\frac{\frac{x + h}{3(x + h) - 1} - \frac{x}{3x - 1}}{h}[/tex]
 
  • #3
I guess it is odd to end up with the right answer.
 
  • #4
Well, Mark44, I have that in the beginning to. I then multiply denominators to get a common one, and I end up with something like:

{ [ (x+h)(3x-1) - x(3x + 3h -1) ] / [ (3x+3h-1)(3x-1) ] } / h

{ ( [3x^2 - x + 3hx - h] - [3x^2 + 3hx -x] ) / [ (3x+3h-1)(3x-1) ] } / h

My problem is that everything cancels out except for h, and that leaves me with only one term, which I know is incorrect. What am I doing wrong here?
 
  • #5
If you write this as x*(3x-1)-1 and do the chain rule you get (3x-1)-1-x*(3x-1)-2*3 which if you put (3x-1)-2 in both denominators gives you (3x-1)/(3x-1)2-(3x)/(3x-1)2 and the 3x cancels out which gives what you got so you got the right answer.
 

Related to Derive x/(3x-1) using first principles

What is "Derive x/(3x-1) using first principles"?

Deriving a function using first principles is a method of finding the derivative of a function by using the definition of a derivative. In this case, we are finding the derivative of the function f(x) = x/(3x-1).

What is the definition of a derivative?

The derivative of a function f(x) is the rate of change of the function at a specific point, represented by f'(x) or dy/dx. It is defined as the limit of the difference quotient as the change in the independent variable approaches zero.

What is the difference quotient?

The difference quotient is a mathematical expression that represents the slope of a secant line between two points on a curve. It can also be used to find the derivative of a function by taking the limit as the change in the independent variable approaches zero.

How do you use first principles to find the derivative of a function?

To find the derivative of a function using first principles, we use the definition of a derivative and the difference quotient. We take the limit of the difference quotient as the change in the independent variable approaches zero. This will give us the slope of the tangent line at a specific point, which is the derivative of the function at that point.

Can you provide the step-by-step process of deriving x/(3x-1) using first principles?

Step 1: Write the difference quotient for the function f(x) = x/(3x-1).
Step 2: Simplify the difference quotient by expanding the numerator and denominator.
Step 3: Take the limit as h (change in independent variable) approaches zero.
Step 4: Substitute 0 for h in the simplified difference quotient.
Step 5: Simplify the resulting expression, which will give us the derivative of f(x).
Step 6: Write the final answer as f'(x) or dy/dx.

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