用函数[z，p，K]=tf2zp（num，den）求得有理分式形式的系统转移函数的零、极点，用函数zplane（z，p）绘出零、极点分布图；-Form of rational fraction obtained with the function [z, p, K] = tf2zp (num, den) system transfer function zero poles, with the function zplane (z, p) draw zero, pole distribution