Finding the First 4 Nonzero Terms of e^tcos(t)

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The discussion focuses on finding the first four nonzero terms of the series expansion for etcos(t). Participants initially consider multiplying the series terms directly but find it inefficient. The recommended approach involves using derivatives to determine the coefficients of the expansion, which simplifies the process significantly. The final terms derived include et(cos(t) - sin(t)), -2et(sin(t)), -2et(cos(t) + sin(t)), and -4et(cos(t)).

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Homework Statement



Find the first 4 nonzero terms of:

e^{t}cos(t)

Homework Equations





The Attempt at a Solution



I am trying to multiply the terms of two known series for my answer, but I'm not sure how to do it efficiently.

Should I list 4 terms of each series, then multiply them as if they were just normal polynomials?

EDIT, I tried doing that and it is completely unfeasible. What's the correct way?
 
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1MileCrash said:

Homework Statement



Find the first 4 nonzero terms of:

e^{t}cos(t)

Homework Equations





The Attempt at a Solution



I am trying to multiply the terms of two known series for my answer, but I'm not sure how to do it efficiently.

Should I list 4 terms of each series, then multiply them as if they were just normal polynomials?

EDIT, I tried doing that and it is completely unfeasible. What's the correct way?

What is "unfeasible" about it? It is unpleasant, maybe, but perfectly feasible. It is not how I would do it, however: I would use derivatives to get the coefficients in the expansion.

RGV
 
I see, so you would take the derivatives evaluated for 0? As the coefficients?

Should I always try that method first?

EDIT: Wait, why would you do it that way? Differentiating that 4 function 4 times would be terrible!
 
1MileCrash said:
Wait, why would you do it that way? Differentiating that 4 function 4 times would be terrible!
It wouldn't be that bad. Give it a try.
 
Mark44 said:
It wouldn't be that bad. Give it a try.

OK. Will report back.
 
OK, it was definitely a lot easier than I anticipated. Thanks again.

1st:
e^t(cost - sint)

2nd:

-2e^t(sin t)

3rd
-2e^t(cos t + sin t)

4th

-4e^t(cos t)
 

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