Differential
Equations Math 285 Hour Exam 3 Fall,
2011
4. |
Solve the following
initial-boundary-value problem: ut = 5 uxx 0
< x <
3π, t
> 0 u(0,t) = u(3π,t) = 0
; u(x,0) = f(x)
∞ using the formula u(x,t) = Σ
bn exp[-k(nπ/L)2]t sin (nπx/L),
where bn
are n=1 suitable constants. a.
f(x) = sin x
+ 10 sin(4x/3) -
sin 2x ∞ b.
f(x) = Σ (100/m2) sin mx m=1 Answers: a. u(x,t) = e-5t sin x + 10
e(-80.9)t sin (4x/3) - e(- 20)t sin 2x ∞ b.
u(x,t)
= Σ (100/m2)
exp( -5m2t) sin mx m=1 |
5. |
Solve the following
initial-boundary-value problem: utt =
100 uxx 0
< x <
5, t
> 0 u(0,t) =
u(5,t) = 0 ;
u(x,0) = f(x),
ut (x,0) =
g(x) Answer: ∞ u(x,t) = Σ [ An cos(2nπt)
+ Bn sin (2nπt) ] sin(nπx/5) n=1 5 where
An = (2/5) ∫ f(x) sin (nπx/5) 0 5 and Bn
= (1/5nπ) ∫ g(x) sin
(nπx/5) 0 |