ddphi.txt

ddphi=
-(((2*a-2)*m*dphi*xl+(2*a-2)*m*dphi*xf
+((2*a-2)*ll0+(2*a-2)*lf0)*m*dphi)*dxl

+((2*a-2)*m*dphi*dxf+(a-1)*m*g*cos(phi))*xl
+(((2*a-2)*ll0-2*lf0)*m*dphi)-2*m*dphi*xf

+(kf*cos(phi)*sin(phi)-df*dphi)*xf^2
(-2*df*lf0*dphi+2*kf*lf0*cos(phi)*sin(phi)-g*m*cos(phi))*xf-df*lf0^2*dphi
+kf*lf0^2*cos(phi)*sin(phi)+((a-1)*g*ll0-g*lf0)*m*cos(phi))
/
((a-1)*m*xl^2+((2*a-2)*m*xf*((2*a-2)*ll0+(2*a-2)*lf0)*m)*xl-m*xf^2
+((2*a-2)*ll0-2*lf0)*m*xf+((a-1)*ll0^2+(2*a-2)*lf0*ll0-lf0^2)*m)


ddphi  | lf0=ll0=0
=- ((2*a-2)*m*dphi*xl+(2*a-2)*m*dphi*xf)*dxl + ((2*a-2)*m*dphi*dxf+(a-1)*m*g*cos(phi))*xl
    - 2*m*dphi*xf*dxf+(kf*cos(phi)*sin(phi)-df*dphi)*xf^2-m*g*cos(phi)*xf
 / 
 ((a-1)*m*xl^2+(2*a-2)*m*xf*xl-m*xf^2)