De Anza College Geometric Progression Series Exercises Discussion

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wvzonat

Mathematics

De Anza College

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(1) Find the radius of convergence for the following series. Show your work.  and interval P∞ 2n 1 n (a) (x − 3) n=1 4 n−1 P∞ (−1) n √ (b) n=1 3n n (x + 2) P∞ n n! (c) n=1 10n n3 (x + 4) (2) Find the function P∞ f (x) whose power series is given in each problem below: (a) f (x) =P n=1 nxn ∞ 1 (b) f(x) = n=1 n+1 xn P∞ (c) f (x) = n=0 an xn where ( 3 if n is divisible by 5 an = 1 otherwise (3) Suppose f (x) = ∞ X kxk k=0 and g (x) = ∞ X k (−1) xk k=0 Find the first first three nonzero terms of the series for the product of the two functions, f (x) g (x). (4) Find the MacLaurin series for f (x) = x3 − 3x2 + 2x + 5 centered at x = 2. (5) Find the power series centered at x = 0 for the following. I would recommend not using the formula for Taylor/MacLaurin series, but manipulating series you already know. 1 (a) f (x) = 1+x P∞3 1 (b) f (x) = k=0 8+x 3 5 (c) f (x) = x2 +x−6 (hint: use the method of partial fractions) x (d) f (x) = 1−cos x2 (6) Suppose that ∞ X k3 k f (x) = (x − 5) k! k=0 Find f (5) (the 100th derivative of f at 5) (7) Use series and/or asymptotic equivalence to evaluate the limit √ 8 1 + x − 8 − 4x + x2 lim x→0 x − sin x Do not use L’Hospital’s rule. (8) (a) Use the binomial theorem to write the first four nonzero terms of the Maclaurin series for Z xp 3 8 + x3 dx (100) 0 (you do not have to simplify numerical expressions) (b) Suppose we want to estimate Z 0.1 p 3 8 + x3 dx 0 How many terms of the series from (a) should we add together to ensure that our sump is within 10−10 = 0.0000000001 of the correct value? 1
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Problem 1. Find the radius R and interval I of convergence for the following series. Show
your work. Each series has the form
X
an (x − x0 )n .
n

(a) The series is a geometric progression series with the ratio
It means that R = 2, I = (3 − 2; 3 + 2) = (1; 5).
(b) The radius of convergence
q

n
3n n = 3.
R = lim


x−3 2
.
2

It converges iff

n→+∞

The endpoints: if x = −5, then the series
n
X 1
X
n−1 (−3)


=

(−1)
n n
3
n
n
n
diverges because 1/2 ≤ 1 (by the integral test); if x = 1, then the series
X
X (−1)n
3n

(−1)n−1 n √ = −
3 n
n
n
n
1
is alternate and converges by the alternate series test, √n+1
< √1n .
So, I = (−5; 1].
(c) The radius of convergence (use the Stirling formula)
s
r
n
3
10e n n5/2
n 10 n
√ = 0.
R = lim
= lim
n→+∞
n→+∞ n
n!


The series converges (to 0) iff x = −2.
2. Find the function f (x) whose power series is given in each problem below.
(a) We have
+∞
d X n
d 1
x
f (x) = x
x =x
=
.
dx n=0
dx 1 − x
(1 − x)2
(b) We have
!
Z
+∞
+∞ Z
+∞
X
xn
1X x n
1 x X n
f (x) =
=
t dt =
t dt
n+1
x n=1 0
x 0
n=1
n=1

Z
Z 
1
−x − ln(1 − x)
1 x t
1 x
=
dt =
...

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