Solve the following initial value problems:
(a) u' + 3u = ex, u(1) = 0
(b) u" + 4u = 1, u(π) = u'(π) = 0
(c) u" - u' - 2u = ex + e-x, u(0) = u'(0) = 0
(d) u" + 2u′ + 5u = sin x, u(0) = 1, u'(0) = 0
(e) u‴ - u" + u' - u = x, u(0) = 0, u'(0) = 1, u"(0) = 0