f1='(U3-U1)/R+(U3-U2)/(p*L)=0'
f2='p*C*U2+(U2-U3)/(p*L)=0'
rU=solve(f1,f2,'U2,U3');
rU.U2
rU.U3
P=rU.U2/'U1'
nP=subs(P,{'R','C','L'},{1e3,22.5e-9,11.25e-3})
[num, den]=numden(nP)
format short e
A=sym2poly(den)
B=sym2poly(num)/A(1)
A=A/A(1)
