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y y = A cos(Ét - kx)
" x
"y "
= (A cos(Ét - kx)) = Ak sin(Ét - kx)
"x "x
"2y "
= (Ak sin(Ét - kx)) = -Ak2 cos(Ét - kx) = -k2y
"x2 "x
" t
"y "
= (A cos(Ét - kx)) = -AÉ sin(Ét - kx)
"t "t
"2y "
= (-AÉ sin(Ét - kx)) = -AÉ2 cos(Ét - kx ) = -É2y
"t2 "t
y
"2y 1 "2y 1
=
"x2 k2 "t2 É2
"2y k2 "2y
=
"x2 É2 "t2
 vfaz = É/k k2/É2
k2 1
=
2
É2 vfaz
"2y 1 "2y
=
"x2 vfaz2 "t2
"2U "2U "2U 1 "2U
+ + =
2
"x2 "y2 "z2 vfaz "t2
U y
1 "2U
"2U =
2
vfaz "t2
"2 "2 "2
"2 = + +
"x2 "y2 "z2

U( = A cos(Ét - k ·
r) r)

k

F
c =
µ

1 1 c 1 F

f = = = =

T   µ
c
L

L = Ò!  = 2L
2

1 F 1 F
f1 = =
 µ 2L µ
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 2L
L = N Ò!  =
2 N
N
fN

1 F N F
fN = =
 µ 2L µ
" F
" µ
©