Question: Differentiation in Lagrangian equation

would you please help me? 

I have a big problem in differentiating with maple! 

here is my equation. 

L := (alpha, beta, theta,alpha(t),beta(t),theta(t))

how can I find diff(L, alpha) ? 

L=(1/2)*I[a]*(diff(alpha(t), t))^2+(1/2)*I[b]*(diff(beta(t), t))^2+(1/2)*I[c]*(diff(theta(t), t))^2+(1/2)*m[3]*(b^2*(diff(beta(t), t))^2+(1/4)*C^2*(diff(theta(t), t))^2+b*c*(diff(beta(t), t))*(diff(theta(t), t))*cos*(beta-theta))-(1/2)*g*m[1]*a*sin*alpha-(1/2)*m[2]*g*b*sin*beta-g*m[3](b*sin*beta+(1/2)*c*sin*theta)-(1/2)*k(sqrt(d^2+2*d*b*cos*beta+2*d*c*cos*theta-2*d*a*cos*alpha+b^2*cos^2*beta^2+2*b*cos^2*beta*c*theta-2*b*cos^2*beta*a*alpha+c^2*cos^2*theta^2-2*c*cos^2*theta*a*alpha+a^2*cos^2*alpha^2+b^2*sin^2*beta^2+2*b*sin^2*beta*c*theta-2*b*sin^2*beta*a*alpha+c^2*sin^2*theta^2-2*c*sin^2*theta*a*alpha+a^2*sin^2*alpha^2)-l[f])^2