Consider the linear homogeneous system x? = p11(t)x + p12(t)y, y? = p21(t)x + p22(t)y. Show that if x = x1(t), y = y1(t) and x = x2(t), y = y2(t) are two solutions of the given system, then x = c1x1(t) + c2x2(t), y = c1y1(t) + c2y2(t) is also a solution for any constants c1 and c2. This is the principle of superposition.