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Sia

mzz's Question
Math
Posted 7 months ago

The Maclaurin series for f(x)f(x) is given by 1+x2!+x23!+x34!++xn(n+1)!+1+\frac{x}{2!}+\frac{x^{2}}{3!}+\frac{x^{3}}{4!}+\cdots+\frac{x^{n}}{(n+1)!}+\cdots
(a) Find f(0)f^{\prime}(0) and f(17)(0)f^{(17)}(0).
(b) For what values of xx does the given series converge? Show your reasoning.
(c) Let g(x)=xf(x)g(x)=x f(x). Write the Maclaurin series for g(x)g(x), showing the first three nonzero terms and the general term.
(d) Write g(x)g(x) in terms of a familiar function without using series. Then, write f(x)f(x) in terms of the same familiar function.
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Answer from Sia
Posted 7 months ago
Solution by Steps
step 1
The Maclaurin series for f(x)f(x) is given by f(x)=1+x2!+x23!+x34!++xn(n+1)!+f(x) = 1 + \frac{x}{2!} + \frac{x^2}{3!} + \frac{x^3}{4!} + \cdots + \frac{x^n}{(n+1)!} + \cdots
step 2
To find f(0)f'(0), we need the coefficient of xx in the series. The coefficient of xx is 12!=12\frac{1}{2!} = \frac{1}{2}. Therefore, f(0)=12f'(0) = \frac{1}{2}
step 3
To find f(17)(0)f^{(17)}(0), we need the coefficient of x17x^{17} in the series. The coefficient of x17x^{17} is 118!\frac{1}{18!}. Therefore, f(17)(0)=118!f^{(17)}(0) = \frac{1}{18!}
Answer
f(0)=12f'(0) = \frac{1}{2}, f(17)(0)=118!f^{(17)}(0) = \frac{1}{18!}


 Part (b)
step 1
The given series is n=0xn(n+1)!\sum_{n=0}^{\infty} \frac{x^n}{(n+1)!}
step 2
To determine the interval of convergence, we use the ratio test
step 3
Compute the ratio an+1an=xn+1(n+2)!(n+1)!xn=xn+2\left| \frac{a_{n+1}}{a_n} \right| = \left| \frac{x^{n+1}}{(n+2)!} \cdot \frac{(n+1)!}{x^n} \right| = \left| \frac{x}{n+2} \right|
step 4
As nn \to \infty, xn+20\left| \frac{x}{n+2} \right| \to 0 for all xx
step 5
Since the ratio test shows that the series converges for all xx, the interval of convergence is (,)(-\infty, \infty)
Answer
The series converges for all x(,)x \in (-\infty, \infty).


 Part (c)
step 1
Let g(x)=xf(x)g(x) = x f(x)
step 2
Substitute the Maclaurin series for f(x)f(x): g(x)=x(1+x2!+x23!+x34!+)g(x) = x \left( 1 + \frac{x}{2!} + \frac{x^2}{3!} + \frac{x^3}{4!} + \cdots \right)
step 3
Distribute xx: g(x)=x+x22!+x33!+x44!+g(x) = x + \frac{x^2}{2!} + \frac{x^3}{3!} + \frac{x^4}{4!} + \cdots
step 4
The first three nonzero terms are xx, x22!\frac{x^2}{2!}, and x33!\frac{x^3}{3!}
step 5
The general term is xn+1(n+1)!\frac{x^{n+1}}{(n+1)!}
Answer
The Maclaurin series for g(x)g(x) is x+x22!+x33!++xn+1(n+1)!+x + \frac{x^2}{2!} + \frac{x^3}{3!} + \cdots + \frac{x^{n+1}}{(n+1)!} + \cdots.


 Part (d)
step 1
We know g(x)=xf(x)g(x) = x f(x)
step 2
From the series, f(x)=ex1f(x) = e^x - 1
step 3
Therefore, g(x)=x(ex1)=xexxg(x) = x (e^x - 1) = x e^x - x
step 4
Thus, f(x)=g(x)+xx=ex1f(x) = \frac{g(x) + x}{x} = e^x - 1
Answer
g(x)=xexxg(x) = x e^x - x, f(x)=ex1f(x) = e^x - 1
Key Concept
Maclaurin series and interval of convergence
Explanation
The Maclaurin series is a representation of a function as an infinite sum of terms calculated from the values of its derivatives at a single point. The interval of convergence is the set of all xx values for which the series converges.

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