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Elementary Differential Equations and Boundary Value Problems - Boyce W.E.

Boyce W.E. Elementary Differential Equations and Boundary Value Problems - John Wiley & Sons, 2001. - 1310 p.
Download (direct link): elementarydifferentialequations2001.pdf
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Applying Theorem 6.3.1 and using Table 6.2.1, we obtain the solution, y = e-tcost + e-tsint - up(t)e (t p)sint.
Note that sin(t-p) = -sint.
3. Taking the Laplace transform and using the I.C. we have
- -10s
2 1 -5s e
(s + 3s+2)Y(s) = + e + ------------. Thus
2s
1/2 e-5s -10s 1/2 1/2 1
Y(s) = ---------- + -------- + e (---------1--------) and hence
s2+3s+2 s2+3s+2 s s+2 s+x
y(t) = h(t) + u5(t)h(t-5) + u10(t)[ + ie-2(t-10)-e-(t-10)]
2 5 10 2 2
where h(t) = e t - e 2t.
5. The Laplace transform of the D.E. is
(s2+2s+3)Y(s) = ----- + e 3ps, so
s2+1
1 3ps 1
Y(s) = ------------------ + e [------------]. Using partial
(s2+1)(s2+2s+3) s2+2s+3
fractions or a computer algebra system we obtain
1 . 1 1 -t r- 1
4 4 . 4
where h(t) = e tsi^V"2t.
y(t) = sint - cost + e cosy 2 t + ;_u3p (t)h(t-3p),
7. Taking the Laplace transform of the D.E. yields
(s2+1)Y(s) - y'(0) = J0 e-st5 (t-2p)costdt. Since
S(t-2p) = 0 for t 2p the integral on the right is equal
-st -2 Ps
to I e o(t-2p) costdt which equals e cos2p from Eq.(16). Substituting for y'(0) and solving for Y(s)
.. -2ps
1e
gives Y(s) = ------- + ----- and hence
22 s2+1 s2+1
sint 0 < t < 2p y(t) = sint + u2p (t)sin(t-2p) = \
[ 2sint 2p < t
10. See the solution for Problem 7.
Section 6.5
117
13a. From Eq. (22) y(t) will complete one cycle when \J15 (t-5)/4 = 2p or T = t - 5 = 8p/V 15 , which is consistent with the plot in Fig. 6.5.3. Since an impulse causes a discontinuity in the first derivative, we need
to find the value of y' at t = 5 and t = 5 + T. From Eq.
(22) we have, for t > 5,
, -(t-5)/4 -1 1
y = e / [-- sin---------- (t-5) + cos-----(t-5)]. Thus
2\j 15 4 24
. 1 . 1 -t/4
y (5) = and y (5+T) = e / . Since the original 22
impulse, 8(t-5), caused a discontinuity in y' of 1/2, we
-T/4
must choose the impulse at t = 5 + T to be -e , which is equal and opposite to y' at 5 + T.
13b. Now consider 2y" + y' + 2y = 8 (t-5) + kd(t-5-T) with
y(0) = 0, y'(0) = 0. Using the results of Example 1 we have
" ,,,e-(t-5)/^^^ ^ 15
y/Ti
+ ^u5+T(t)e-(t-5-T)/4si^^^(t-
V 15 4
e (t 5)/4 [u5 (t)sin ^ 5 (t-5) +ku5+T (t)eT/4sin^ 5_ į 5 4 5+t 4
y(t) = < u5(t)e / sin------ (t-5)
e (t 5)/4[u5(t)+keT/4u5+T(t)]sin-----------------------(t-5). If
\[
y(t) = 0 for t > 5 + T, then 1 + keT/4 = 0, or k = -e T/4, as found in part (a).
20 20
_ -ks
kps 1 e
e so that Y(s) =
e kps so that Y(s) = ^
k=1 k=1 s +1
20
and hence y(t) = ukp (t)sin(t-kp)
k=1
= up (t)sin(t-p) + u2p (t)sin(t-2p) + ... + u10psin(t-10p). For 0 < t < p, y(t) = 0. For p < t < 2p, y(t) = sin(t-p) = -sint. For 2p < t < 3p,
y(t) = sin(t-p) + sin(t-2p) = -sint + sint = 0. Due to the periodicity of sint, the solution will exhibit this behavior in alternate intervals for 0 < t < 20p. After t = 20p the solution remains at zero.
21b. Taking the transform and using the I.C. we have
118
Section 6.6
15 15
e-(2k-1)p
(s2 + 1)Y(s) = e-(2k-1)p so that Y(s) = ----- -------
k=1 k=1 s +1
15
Thus y(t) = X u (2k-1)p (t)sin[t-(2k-1)p]
k=1
= sin(t-p) + sin(t-3p) ... + sin(t-29p)
= -sint - sint ... -sint = -15sint.
25b. Substituting for f(t) we have
y = j0e-(t-t * 8 (x-p)sin(t-T)dx. We know that the
integration variable is always less than t (the upper limit) and thus for t < p we have t < p and thus 8(t-p) = 0. Hence y = 0 for t < p. For t > p utilize Eq.(16).
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