Help with simplifying derivatives when sketching graphs

In summary, the conversation is about finding the derivative and second derivative of the function x^(4/9)*e^(-x) and using both the sum and factored form of the derivative to simplify the second derivative. The conversation also touches on the usefulness of both forms in different situations and the use of the quadratic formula to factor the resulting quadratic.
  • #1
TsAmE
132
0

Homework Statement



Sketch the graph of x ^ (4/9) * e ^ (-x)

Homework Equations



None.

The Attempt at a Solution



My y' = -x ^ (4/9) * e ^ (-x) ( 1 - 4/9x ^ 1/9). I keep on getting a reaaally long derivative for y'' and thus cannot place it on my sign table. Could someone please show me the correct steps in order to get a simplified y''?
 
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  • #2
TsAmE said:

Homework Statement



Sketch the graph of x ^ (4/9) * e ^ (-x)

Homework Equations



None.

The Attempt at a Solution



My y' = -x ^ (4/9) * e ^ (-x) ( 1 - 4/9x ^ 1/9). I keep on getting a reaaally long derivative for y'' and thus cannot place it on my sign table. Could someone please show me the correct steps in order to get a simplified y''?

You can write y' in two ways - as a sum from the product rule, or as a product. Each form is useful for some purpose.

If y = x4/9e-x,
y' = (4/9)x-5/9 e-x - x4/9 e -x ;; as a sum (actually a difference) straight from the product rule
= e-x x-5/9 (4/9 - x) ;; in factored form

The first form is probably easier to differentiate so that you can get y''. The factored form is more useful if you want to find critical numbers and intervals where y' > 0 or y' < 0.

I think your derivative has an error in it.
 
  • #3
Oh didnt see that mistake thanks. The only problem I am having is finding y'' since you have to differentiate the sum of 2 products or 3 products which has led me to getting y'' to be almost 2 lines long.
 
  • #4
If you start with the first form I showed above (the difference, not the factored form), you should be able to write y'' with at most four terms.
 
  • #5
Oh ok I did as you said now I got a nice simplified

y'' = x^(-14/9) * e^(-x) * ( x^(2) - 8/9x - 20/81 )

but I have no idea how I could factorise the quadratic due to the fractions? Unless I have made some mistake, although I doubt it, cause I checked rigorously
 
  • #6
Your expression for y'' look fine to me. To factor the quadratic, it's probably most efficient to use the quadratic formula.
 

What is the purpose of simplifying derivatives when sketching graphs?

Simplifying derivatives when sketching graphs is important because it helps us to understand the overall behavior of a function and how it changes at different points. It also allows us to identify key features, such as maximum and minimum points, and inflection points.

What are some common techniques for simplifying derivatives?

Some common techniques for simplifying derivatives include using the power rule, product rule, quotient rule, and chain rule. It is also helpful to know basic algebraic simplification techniques, such as factoring and canceling common terms.

How can simplifying derivatives help us to identify key features of a graph?

By simplifying derivatives, we can find the critical points of a function, which are points where the derivative is equal to zero or undefined. These critical points can tell us where a graph has maximum or minimum values, as well as inflection points where the concavity of the graph changes.

What are some common mistakes to avoid when simplifying derivatives?

One common mistake is to forget to apply the chain rule when differentiating composite functions. It is also important to be careful with algebraic simplification and to double-check your work for any potential mistakes or errors.

How can I improve my skills in simplifying derivatives when sketching graphs?

Practice is key! Make sure to fully understand the basic derivative rules and how they apply to different types of functions. Additionally, it can be helpful to work on a variety of examples and to seek out additional resources, such as textbooks or online tutorials, for extra practice and guidance.

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