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Find the first four nonzero terms in the Maclaurin series for [tex]f(x)[/tex].

Given: [tex]f(x) = e^{6x} \sin x[/tex]

Choose the correct answer below:

A. [tex]x + 6x^2 - \frac{109}{6}x^3 - \frac{6121}{120}x^5 + \cdots[/tex]

B. [tex]x + 6x^2 + \frac{107}{6}x^3 + 35x^4 + \cdots[/tex]

C. [tex]1 + x + 6x^2 + \frac{107}{6}x^3 + \cdots[/tex]

D. [tex]1 + 6x + 6x^2 - \frac{109}{6}x^3 + \cdots[/tex]

Answer :

We want to find the first four nonzero terms of the Maclaurin series of

[tex]$$
f(x) = e^{6x} \sin x.
$$[/tex]

The Maclaurin series expansion of a function is its power series expansion about [tex]\( x=0 \)[/tex].

Step 1. Write the known series for each factor.

The series for the exponential function is

[tex]$$
e^{6x} = 1 + 6x + \frac{(6x)^2}{2!} + \frac{(6x)^3}{3!} + \frac{(6x)^4}{4!} + \cdots
= 1 + 6x + 18 x^2 + 36 x^3 + 54 x^4 + \cdots.
$$[/tex]

The series for the sine function is

[tex]$$
\sin x = x - \frac{x^3}{3!} + \frac{x^5}{5!} - \cdots
= x - \frac{x^3}{6} + \frac{x^5}{120} - \cdots.
$$[/tex]

Step 2. Multiply the series for [tex]\(e^{6x}\)[/tex] and [tex]\(\sin x\)[/tex].

Since the series for [tex]\(\sin x\)[/tex] begins with [tex]\(x\)[/tex], the lowest degree term in the product will be of degree 1.

We expand carefully to find the terms up to [tex]\(x^4\)[/tex]:

1. Term in [tex]\(x\)[/tex]:

Multiply the constant in [tex]\(e^{6x}\)[/tex] by the [tex]\(x\)[/tex] term in [tex]\(\sin x\)[/tex]:

[tex]$$
1 \cdot x = x.
$$[/tex]

2. Term in [tex]\(x^2\)[/tex]:

Multiply the [tex]\(6x\)[/tex] term (from [tex]\(e^{6x}\)[/tex]) by the [tex]\(x\)[/tex] term (from [tex]\(\sin x\)[/tex]):

[tex]$$
6x \cdot x = 6x^2.
$$[/tex]

3. Term in [tex]\(x^3\)[/tex]:

There are two contributions:

- The constant [tex]\(1\)[/tex] from [tex]\(e^{6x}\)[/tex] times the [tex]\(-\frac{x^3}{6}\)[/tex] term from [tex]\(\sin x\)[/tex]:

[tex]$$
1 \cdot \left(-\frac{x^3}{6}\right) = -\frac{x^3}{6}.
$$[/tex]

- The [tex]\(18x^2\)[/tex] term from [tex]\(e^{6x}\)[/tex] times the [tex]\(x\)[/tex] term from [tex]\(\sin x\)[/tex]:

[tex]$$
18x^2 \cdot x = 18x^3.
$$[/tex]

Summing the contributions, we obtain:

[tex]$$
-\frac{x^3}{6} + 18x^3 = \left(18 - \frac{1}{6}\right)x^3 = \frac{107}{6} x^3.
$$[/tex]

4. Term in [tex]\(x^4\)[/tex]:

The contributions are:

- The [tex]\(6x\)[/tex] term from [tex]\(e^{6x}\)[/tex] times the [tex]\(-\frac{x^3}{6}\)[/tex] term from [tex]\(\sin x\)[/tex]:

[tex]$$
6x \cdot \left(-\frac{x^3}{6}\right) = -x^4.
$$[/tex]

- The [tex]\(36x^3\)[/tex] term from [tex]\(e^{6x}\)[/tex] times the [tex]\(x\)[/tex] term from [tex]\(\sin x\)[/tex]:

[tex]$$
36x^3 \cdot x = 36x^4.
$$[/tex]

Adding these, we get:

[tex]$$
-x^4 + 36x^4 = 35x^4.
$$[/tex]

Step 3. Write the final answer.

The first four nonzero terms in the Maclaurin series for [tex]\( f(x)=e^{6x} \sin x \)[/tex] are

[tex]$$
x + 6x^2 + \frac{107}{6}x^3 + 35x^4 + \cdots.
$$[/tex]

Comparing with the given options, the correct answer is:

B. [tex]\(x+ 6x^2+ \frac{107}{6}x^3+ 35x^4+\cdots\)[/tex].

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