Finding nth derivative of function

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Homework Help Overview

The discussion revolves around finding a general formula for the nth derivative of the function y = e^(x) cos(x). Participants are exploring the derivatives and attempting to identify a pattern that could lead to a formula for y^(k) where k is a positive integer.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants are examining the derivatives of the function and discussing the results of their calculations. Some suggest focusing on even derivatives, while others propose calculating additional derivatives to identify a pattern. There are questions about the existence of a function that could represent the nth derivative.

Discussion Status

The discussion is ongoing, with various participants sharing their findings and observations about the derivatives. Some have noted the doubling pattern in coefficients and the alternating nature of sine and cosine, while others express uncertainty about the signs and overall pattern. There is no explicit consensus yet, but several productive lines of inquiry are being explored.

Contextual Notes

Participants are grappling with the signs of the derivatives and how they relate to the overall pattern. There is mention of a cycle repeating every four derivatives, which is being considered in the context of forming a general formula.

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Homework Statement



For each n, let y^(n) denote the nth derivative of the function y = e^(x) cosx.

Can you suggest a general formula for y^(k), where k is an interger > 0? What is y^(2010)?

Homework Equations



None

The Attempt at a Solution



I got e^(x) (cosx - sinx) for y'

-2e^(x) sinx for y''

-2e^(x) (cosx + sinx) for y''' and

-4e^(x) cosx for y''''

which are right, but I don't see a pattern for the nth derivative, which could help me answer the question.
 
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Start by looking at the even derivatives.
 
I would suggest doing another 4 derivatives, and then see if you can notice the pattern. Often times when I am looking for a pattern in data, more data really helps.

Is there a name for a function than takes in some natural number n and gives the n'th derivative?
 
Cyosis said:
Start by looking at the even derivatives.

I got:

y^(6) = 8e^(x) sinx

y^(8) = 16e^(x) cosx

but still don't see a pattern in comparison to:

-2e^(x) sinx for y''
-4e^(x) cosx for y''''

as it isn't alternating negative/positive sign or having a constant sign, but otherwise it alternating between cosx and sinx and is doubling
 
Take the derivative of cos x four times and you will see the pattern when it comes to the minus signs. Yes it is doubling you should be able to link this to 2^n.
 
But for y(2), y(4), y(6) and y(8) the signs are +, +, -, -. I can't see a pattern to represent these in an nth formula
 
The cycle repeats every 4th derivative, so you might find it helpful to think about how to express y^(4m), y^(4m+1), y^(4m+2) and y^(4m+3). For example, you found that

y^(0) = ex cos x = (-4)0 cos x
y^(4) = -4 ex cos x = (-4)1 cos x
y^(8) = 16 ex cos x = (-4)2 cos x

so you can say that y^(4m) = (-4)m cos x.
 
But for y(2), y(4), y(6) and y(8) the signs are +, +, -, -. I can't see a pattern to represent these in an nth formula

No y(0),y(2),y(4),y(6) are +,-,-,+. Have you taken the derivative of the cosine four times yet?
 

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