dsolve({diff(x1(t),t) = a*x2(t)+b*x3(t)*cos(c*t)+b*x4(t)*sin(c*t), diff(x2(t),t) = -a*x1(t)+b*x3(t)*sin(c*t)-b*x4(t)*cos(c*t), diff(x3(t),t) = -b*x1(t)*cos(c*t)-b*x2(t)*sin(c*t)+a*x4(t), diff(x4(t),t) = -b*x1(t)*sin(c*t)+b*x2(t)*cos(c*t)-a*x3(t)})