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
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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:
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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