Finding the Derivative of y=sqrt(x+sqrt(x+sqrt(x)))

In summary: I will try to replicate it and post back.In summary, my attempted solution is dy/dx=1/(2sqrt(x+sqrt(x+sqrtx)))*(1+(1/(2sqrt(x+sqrtx))))*(1+(1/(2sqrtx))))
  • #1
ScienceMan
12
1

Homework Statement


This is a chain rule problem that I can't seem to get right no matter what I do. It wants me to find the derivative of y=sqrt(x+sqrt(x+sqrt(x)))

Homework Equations


dy/dx=(dy/du)*(du/dx)
d/dx sqrtx=1/(2sqrtx)
d/dx x=1
(f(x)+g(x))'=f'(x)+g'(x)

The Attempt at a Solution


My attempted solution is dy/dx=1/(2sqrt(x+sqrt(x+sqrtx)))*(1+(1/(2sqrt(x+sqrtx))))*(1+(1/(2sqrtx))).

I took the derivative of the outermost function and left the inner function alone, then multiplied by the derivative of the inner function, and continuing until I reached the innermost function. I'm doing exactly what I was told to do in class and this approach has worked on all the other problems so far, but my online homework system isn't taking this as a solution. Where am I going wrong?
 
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  • #2
What result did you get (what are submitting for an answer). Can you outline your steps? Have you checked against something like Wolframalpha?
 
  • #3
scottdave said:
What result did you get (what are submitting for an answer). Can you outline your steps? Have you checked against something like Wolframalpha?
I'm submitting exactly what I put in for the answer I got. I know formatting isn't the problem, since it would tell me if it was (I'm using WebWorks in case that matters.) I did check it against Wolfram Alpha, and you can see the result here. As far as I can tell the results are the same, but I might be wrong on that as WA is giving the answer in an odd format instead of showing the factors being multiplied side by side.

My steps were:

1. Take the derivative of y with respect to u of the outermost square root, with u being everything inside that, giving me 1/(2sqrt(x+sqrt(x+sqrtx)))
2. Take the derivative of u with respect to x, with a new u being anything inside the 2nd most outer square root, giving me (1+(1/(2sqrt(x+sqrtx))))
3. Take the derivative of the new u with respect to x, giving me (1+(1/(2sqrtx)))
4. Multiply them all together.
 
  • #4
I think the step 4. multiply all together, may be where you might have issues. It's a little vague. What answer did you arrive at?
Try working from the inside out:

We have y=sqrt(x+sqrt(x+sqrt(x)))
set u = x+sqrt(x), so du/dx = (1 + 1/(2sqrt(x)))
set v = x + sqrt(u), so dv/dx = { 1 + 1/(2sqrt(u)) }*(du/dx)
Now y = sqrt(v). so dy/dx = 1/(2sqrt(v)) * (dv/dx)
Substitute in for u, v and du/dx and dv/dx.

You could try picking different values for x, and then estimating the slope by calculating Δy / Δx, picking sufficiently small values for Δx.
See how these values compare with your expression for dy/dx.

If all of that agrees, and the automated homework system still won't accept it, then it's time to contact your instructor and see if they provide guidance.
 
  • #5
Here are some values that I got for Δy / Δx, using Δx = 1 x 10-5:
x = 1, Δy / Δx = 0.4925
x = 2, Δy / Δx = 0.3483
x = 3, Δy / Δx = 0.2849
x = 4, Δy / Δx = 0.2471
x= 10, Δy / Δx = 0.1571

These are plugging into the your original expression for y, not even doing the derivative. If your expression for derivative is correct, then you should be able to plug in x values and get close to these numbers for dy/dx.
 
  • #6
ScienceMan said:

Homework Statement


This is a chain rule problem that I can't seem to get right no matter what I do. It wants me to find the derivative of y=sqrt(x+sqrt(x+sqrt(x)))

Homework Equations


dy/dx=(dy/du)*(du/dx)
d/dx sqrtx=1/(2sqrtx)
d/dx x=1
(f(x)+g(x))'=f'(x)+g'(x)

The Attempt at a Solution


My attempted solution is dy/dx=1/(2sqrt(x+sqrt(x+sqrtx)))*(1+(1/(2sqrt(x+sqrtx))))*(1+(1/(2sqrtx))).

I took the derivative of the outermost function and left the inner function alone, then multiplied by the derivative of the inner function, and continuing until I reached the innermost function. I'm doing exactly what I was told to do in class and this approach has worked on all the other problems so far, but my online homework system isn't taking this as a solution. Where am I going wrong?

You are doing nothing wrong; if the answer you submitted is exactly what you have typed here then it should be acceptable, unless there are some unbalanced parentheses----always a possibility when you have so many nested together like you do. As others have suggested, maybe there is also a formatting issue.
 
  • #7
Maybe I missed your typed out solution before? I was on my phone when I originally saw your post. I looked at your expression. I am not sure if your expression is right though.
 
  • #8
I figure I should post the end result of this. I ended up asking my instructor and the problem was I needed to move some parentheses around. I guess I messed those up somehow and it ended up screwing up the answer somehow. Thanks for the help everyone.
 

1. What is the process for finding the derivative of y=sqrt(x+sqrt(x+sqrt(x)))?

The process for finding the derivative of this function involves using the chain rule and the power rule. First, we rewrite the function as y = (x + (x + (x)^(1/2))^(1/2))^(1/2). Then, we take the derivative of the innermost function, which is (x)^(1/2), using the power rule. Next, we use the chain rule to find the derivative of the next layer, (x + (x)^(1/2))^(1/2). Finally, we use the power rule again to find the derivative of the outermost layer, (x + (x + (x)^(1/2))^(1/2))^(1/2).

2. Why is the chain rule necessary for finding the derivative of this function?

The chain rule is necessary because this function is composed of multiple nested functions. The chain rule allows us to find the derivative of the outermost function by taking into account the derivatives of the inner functions.

3. How do I know when to use the power rule and when to use the chain rule in finding the derivative of this function?

You can use the power rule whenever you have a function raised to a constant power. You can use the chain rule whenever you have a function inside another function. In this case, we use the power rule for the innermost function and the chain rule for the outer layers.

4. What are the steps for applying the power rule in finding the derivative of y=sqrt(x+sqrt(x+sqrt(x)))?

The steps for applying the power rule in this function are as follows: first, we bring the exponent down and subtract 1 from it to get (1/2)-1 = -1/2. Then, we multiply this by the derivative of the inside function, which is (1/2)x^(-1/2). Finally, we simplify to get the derivative as (1/2)x^(-3/2).

5. Can I simplify the derivative of y=sqrt(x+sqrt(x+sqrt(x))) further?

Yes, you can further simplify the derivative by multiplying the constant 1/2 with the negative exponent to get the final derivative as 1/(2sqrt(x(x+sqrt(x)))). However, this is not a required step and the derivative can be left in its original form as (1/2)x^(-3/2).

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