11. f(z) = -z -L
Advanced Engineering Mathematics
10th Edition
ISBN:9780470458365
Author:Erwin Kreyszig
Publisher:Erwin Kreyszig
Chapter2: Second-order Linear Odes
Section: Chapter Questions
Problem 1RQ
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I really confused about how to solve this type of question. How do we know the coefficient of a0, am and bm is 1/L? and what is the answer for a0 from the -L to L?
![11. f (x) :
= -x -L< x < L; f(x+2L) = f (x)
Answer
V Solution
(a) The figure shows the case L = 1.
(b) The Fourier series is of the form
ao
MTL
f(x)
2
+E l
+ bm sin
am cos
L
L
т-1
where the coefficients are computed form Eqs.(8)-(10). Substituting for f(x) in these equations yields
(1/L) / (-x) dæ = 0
1
Am =
L
and
(-а) cos
dx = 0,
L
--
-L
m = 1, 2, ... (these can be shown by direct integration, or using the fact that
g(x) dæ
= 0 when g(x) is an odd function). Finally,
-a
1
тт
1
(7)
bm
sin
dx =
COS
CoS
dx
т
--
тт
L
-L
2L cos mT
2L(-1)"
sin
тт
m²72
т
since cos mT = (-1)". Substituting these terms in the above Fourier series for f(x) yields the desired answer:](/v2/_next/image?url=https%3A%2F%2Fcontent.bartleby.com%2Fqna-images%2Fquestion%2F81d7096f-8065-4480-a939-1687e3ea3984%2Fcac6d638-f8cc-4ea8-8e78-62e6f4b31293%2Fe9qi3ko_processed.png&w=3840&q=75)
Transcribed Image Text:11. f (x) :
= -x -L< x < L; f(x+2L) = f (x)
Answer
V Solution
(a) The figure shows the case L = 1.
(b) The Fourier series is of the form
ao
MTL
f(x)
2
+E l
+ bm sin
am cos
L
L
т-1
where the coefficients are computed form Eqs.(8)-(10). Substituting for f(x) in these equations yields
(1/L) / (-x) dæ = 0
1
Am =
L
and
(-а) cos
dx = 0,
L
--
-L
m = 1, 2, ... (these can be shown by direct integration, or using the fact that
g(x) dæ
= 0 when g(x) is an odd function). Finally,
-a
1
тт
1
(7)
bm
sin
dx =
COS
CoS
dx
т
--
тт
L
-L
2L cos mT
2L(-1)"
sin
тт
m²72
т
since cos mT = (-1)". Substituting these terms in the above Fourier series for f(x) yields the desired answer:
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