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

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In summary, to find the first 4 nonzero terms of e^{t}cos(t), you can use derivatives to get the coefficients in the expansion, which would be -4e^t(cos t), -2e^t(cos t + sin t), -2e^t(sin t), and e^t(cos t - sin t).
  • #1
1MileCrash
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Homework Statement



Find the first 4 nonzero terms of:

[itex]e^{t}cos(t)[/itex]

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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  • #2
1MileCrash said:

Homework Statement



Find the first 4 nonzero terms of:

[itex]e^{t}cos(t)[/itex]

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
 
  • #3
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!
 
  • #4
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.
 
  • #5
Mark44 said:
It wouldn't be that bad. Give it a try.

OK. Will report back.
 
  • #6
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)
 

What is e^tcos(t)?

e^tcos(t) is a mathematical expression that represents the exponential function e^t multiplied by the cosine function cos(t).

What are the first 4 nonzero terms of e^tcos(t)?

The first 4 nonzero terms of e^tcos(t) are e^t, te^tcos(t), t^2e^tcos(t), and t^3e^tcos(t).

Why is it important to find the first 4 nonzero terms of e^tcos(t)?

Finding the first 4 nonzero terms of e^tcos(t) allows us to approximate the value of the function for small values of t, which can be useful in various scientific and mathematical applications.

How can I calculate the first 4 nonzero terms of e^tcos(t)?

To calculate the first 4 nonzero terms of e^tcos(t), you can use the Taylor series expansion of the function, which involves taking derivatives of e^tcos(t) and evaluating them at t=0.

What are some real-world applications of e^tcos(t)?

The function e^tcos(t) has applications in various fields such as physics, engineering, and economics. For example, it can be used to model the behavior of a damped oscillator in physics or to predict the growth of a population in biology.

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