说明：  使用“改进的欧拉公式”和“4阶龙格-库塔公式”分别对Volterra方程求解，绘制解曲线、相轨线，并将结果进行比较。
(Using the " improved Euler formula" and " 4-order Runge- Kutta formulas," respectively, Volterra equations, draw solution curves, phase trajectories, and the results were compared.)