tt xt + (7 The generalized method of separation of variables can also be applie differential equations. We now show this using the example Wtt + (27)wet +(7² – 1)wrx 0,

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8.2.3
Wit + (27)Wat + (y² – 1)wæx
The generalized method of separation of variables can also be applied to partial
differential equations. We now show this using the example
Wtt + (27)wxt +(y² – 1)wrx = 0,
(8.40)
where w =
w(x, t), wt = ôw/Ət, wx = dw/dx, etc.
Assume w(x, t) to have the form
w(x, t) = X(x)T(t).
(8.41)
Substitute into equation (8.40) and divide by X(x)T(t), to obtain
X'
+ 2y
X
+ (y² – 1)
- 0,
(8.42)
X
where the primes denote differentiation with respect to the appropriate vari-
able, x or t. Now, take
T'
E= -a?, a = arbitrary constant,
(8.43)
an equation whose solution is
(T(t, a?) = A(a²)e¬«'t,
(8.44)
where A(a2) is a function of a². Substituting this result into equation (8.42)
and simplifying gives
-22)X" + (2a²)X' – a*X = 0.
(8.45)
The characteristic equation is
((1- 72)r2 + (2a²)r – a* = 0,
(8.46)
and it has the roots
a?
a2
r1
(8.47)
T2 = -
1+y'
Therefore,
(x(2, a²) = B1(a®)e(#)-
+ B2(a²)e¬()=.
1+y
X (x,
(8.48)
and w(x, t, a²) = X (x, a²)T(t, a²) is
W (x, t, a²) = A1(a²) exp
+ Azla*) exp{- [(1,)
x + a²t
(8.49)
If we now sum/integrate over a², the following result is obtained for the general
solution to equation (8.40)
w(x, t) = f[x – (1+y)t] + g[x + (1 – )t],
(8.50)
where f(z) and g(z) are arbitrary functions of z.
Transcribed Image Text:8.2.3 Wit + (27)Wat + (y² – 1)wæx The generalized method of separation of variables can also be applied to partial differential equations. We now show this using the example Wtt + (27)wxt +(y² – 1)wrx = 0, (8.40) where w = w(x, t), wt = ôw/Ət, wx = dw/dx, etc. Assume w(x, t) to have the form w(x, t) = X(x)T(t). (8.41) Substitute into equation (8.40) and divide by X(x)T(t), to obtain X' + 2y X + (y² – 1) - 0, (8.42) X where the primes denote differentiation with respect to the appropriate vari- able, x or t. Now, take T' E= -a?, a = arbitrary constant, (8.43) an equation whose solution is (T(t, a?) = A(a²)e¬«'t, (8.44) where A(a2) is a function of a². Substituting this result into equation (8.42) and simplifying gives -22)X" + (2a²)X' – a*X = 0. (8.45) The characteristic equation is ((1- 72)r2 + (2a²)r – a* = 0, (8.46) and it has the roots a? a2 r1 (8.47) T2 = - 1+y' Therefore, (x(2, a²) = B1(a®)e(#)- + B2(a²)e¬()=. 1+y X (x, (8.48) and w(x, t, a²) = X (x, a²)T(t, a²) is W (x, t, a²) = A1(a²) exp + Azla*) exp{- [(1,) x + a²t (8.49) If we now sum/integrate over a², the following result is obtained for the general solution to equation (8.40) w(x, t) = f[x – (1+y)t] + g[x + (1 – )t], (8.50) where f(z) and g(z) are arbitrary functions of z.
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