\fIn Problems 12 and 13, find the natural frequencies of the three-mass system of Fig. 7.5.1 2, using the given masses and spring constants. For each natural frequency w, give the ratio a1:a2:a3 of amplitudes for a corresponding natural mode x1 = a1 cos wt, x2 = a2 cos wt, x3 = a3 cos wt. 12. m1 = m2 = m3 = 1; k1 = k2 = k3 = k4 = 1 13. m1 = m2 = m3 = 1; k1 = k2 = k3 = k4 = 2 (Hint: One eigenvalue is ) = -4.)