操作方法
（01）>> syms x y;>> df_dx=diff(cos(x+sin(y))-sin(y),x)df_dx =-sin(x + sin(y))>> df_dy=diff(cos(x+sin(y))-sin(y),y)df_dy =- cos(y) - sin(x + sin(y))*cos(y)>> dy_dx=-df_dx/df_dydy_dx =-sin(x + sin(y))/(cos(y) + sin(x + sin(y))*cos(y))
（02）>> pretty(dy_dx)sin(x + sin(y))- -------------------------------cos(y) + sin(x + sin(y)) cos(y)
