# symbolic derivative of lagrangian & kinematic coefficient

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Young Lee on 7 May 2020
Answered: Guru Mohanty on 11 May 2020
Hi
II am working on some mechanical problems.
basically trying to take symbolic derivative of kinematic coefficients for the purpose of checking my hand calculations..
so here is the position scaler ; r2 cos(th2) - r3 * cos(th3) = 0
taking derivative with respect to th2 (theta2) , Vx = -r2 *sin(th2) + r3 *sin(th3) * d(th3)/d(th2) - cos(th3)*d(r3)/d(th2) = 0
and differentiate again for acceleration with respect to th2
Could you please explain on how this can be achieved using matlab?

Guru Mohanty on 11 May 2020
Hi, I understand you want to find the first and second derivative of the following expression. You can do this by the following steps.
1. Declare variables using syms.
2. Build the expression.
3. Define the first and second derivative using diff function.
Here is a sample code for it.
syms th2 r2 th3(th2) r3(th2)
disp = r2*cos(th2) - r3*cos(th3);
vel=diff(disp,th2);
accel=diff(vel,th2);