Let u(x,t) is a function, which describes vibration of string. So we can rewrite task in the next form:
∂t2∂2u=b2∂x2∂2u
u(0,t)=u(1,t)=0
u(x,0)=f(x)=a(x−x2)
∂t∂u∣t=0=0
To solve this task I use a Fourier method.
Let:
u(x,t)=X(x)∗T(t)
Then,
T′′X=b2TX′′
b2TT′′=XX′′=const=−λ2
So we have two tasks:
1)X′′+λ2X=0
2)T′′+(bλ)2T=0
For the task 1) we have 2 boundary conditions:
u(0,t)=X(0)T(t)=0=>X(0)=0
u(1,t)=X(1)T(t)=0=>X(1)=0
The first task solution is:
X(x)=Acos(λx)+Bsin(λx)
From the first boundary condition we have that A=0. The second give us:
X(1)=Bsin(λ)=0=>λ=πn=>X(x)=Bsin(πnx)
But we know that:
∣∣X(x)∣∣=1=>
1∫0∣X(x)∣2dx=1∫0B2sin2(πnx)dx=B2∗21=1
B=2
For the task 2) we have next solution:
T(t)=Ccos(λbt)+Dsin(λbt)=Ccos(πbnt)+Dsin(πbnt)
The result of the main task is:
u(x,t)=2i=1∑∞(Ccos(πnbt)+Dsin(πnbt))sin(πnx)
But we also have two initial conditions. The first one:
u(x,0)=2i=1∑∞Csin(πnx)=f(x)
Multiply both side by sin(πkx):
f(x)sin(πkx)=2i=1∑∞Csin(πnx)sin(πkx)
Now integrate both sides with respect to x:
∫01f(x)sin(πkx)dx=2∫01Csin(πnx)sin(πkx)dx
Because trigonomical function are ortogonal, the right side give us:
{01/2if k=/nif k=n
So
C=2∫01f(x)sin(πnx)dx
C=2a∫01(x−x2)sin(πnx)dx=−(πn)32acos(πnx)∣10=−(πn)32a(cos(πn)−1)=(π(2n−1))34a
The second initial condition:
∂t∂u∣t=0=2πnai=1∑∞Dsin(πnx)=0=>D=0
So the answer is:
u(x,t)=π34a2i=1∑∞(2n−1)31cos(π(2n−1)bt)sin(π(2n−1)x)
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