如何将带符号变量的状态空间矩阵转换成传递函数？

[home_rainbow]

syms K1 K2 C1 C2 C3 real
syms J1 J2 J3 positive
A=[0,1,0,0,0,0;-K1/J1,-C1/J1,K1/J1,0,0,0;0,0,0,1,0,0;K1/J2,0,-(K1+K2)/J2,-C2/J2,K2/J2,0;0,0,0,0,0,1;0,0,K2/J3,0,-K2/J3,-C3/J3];
B=[0;1/J1;0;0;0;0];
C=[1,0,0,0,0,0;0,0,1,0,0,0;0,0,0,0,1,0];
D=[0;0;0];
[num den]=ss2tf(A,B,C,D);

我写了上面的代码可是不对， 请问应该怎么写呢？