Solve each of the linear systems in Problems 13 through 20 to determine whether the critical point 0 0 is stable asymptot cally stable or unstable Use a computer system or graphing calculator to construct a phase portrait and direction field for the given system Thereby ascertain the stability or instabil ity of each critical point and identify it visually as a node a saddle point a center or a spiral point 15 dx dt 2x dy dt y