Mathematica help - diferential eq

Hi! I want to compute a partial derivative of the equation (please find it
attached) with respect to the variable x:

ImageShack - Hosting :: diffeqyi0.png

How can i do that with Mathematica? Many thanks

I think what you will want to do is to write x as x[t] and j as j[t], so the equation would be:
x''[t]Š1/(M+m Sin[j[t]]^2) *(m Sin[j[t]] (g Cos[j[t]]+ l j'[t]^2)-D x'[t]+F u)
and to take the partial derivative with respect to x, you would write:

D[x''[t]Š1/(M+m Sin[j[t]]^2) *(m Sin[j[t]] (g Cos[j[t]]+ l j'[t]^2)-D x'[t]+F u),x]

which gives

x(3)[t]=(D x''[t])/(M+m Sin[j[t]]2)