t1 = 
Hi Yusuf,
I understand you want to get the coefficient values of all the terms which contains thetadot1*thetadot2.
You can achieve this using "coeffs" function. Here is the code to get you started:
syms m1 m2 l1 l2 thetadotdot1 thetadotdot2 thetadot1 thetadot2 theta1 theta2 g; n=1;
t1=m2*l2^2*(thetadotdot1+thetadotdot2)+m2*l1*l2*cos(theta2)*(2*thetadotdot1+thetadotdot2)+(m1+m2)*l1^2*thetadotdot1-m2*l1*l2*sin(theta2)*(thetadot2^2)-2*m2*l1*l2*sin(theta2)*(thetadot1*thetadot2)+m2*l2*g*cos(theta1+theta2)+(m1+m2)*l1*g*cos(theta1)
target_term = thetadot1 * thetadot2;
t1_factored = factor(t1);
[coeffs_array, terms_array] = coeffs(t1_factored, [thetadot1, thetadot2]);
index = find(terms_array == target_term);
coefficient = coeffs_array(index)
Refer to the following MathWorks documentation for more information on "coeffs" function:
