Question: how to solve this simplification problem?

hi guys i have a question for you,

can anyone help me to find the correct code to mantain the structure of vel_qD list and simplify the term diff(s1(t),t)??

i'm driving crazy for that beacuse i'm not able to do it...

thanks, down here i post the code...

vel_qD := [diff(theta1(t), t) = -(diff(s1(t), t))*cos(theta3(t))/(LAD*sin(theta1(t)+theta3(t))), diff(theta2(t), t) = (L1*sin(theta1(t)+theta3(t))*(diff(s1(t), t))+L1*sin(theta1(t)-theta3(t))*(diff(s1(t), t))-2*LAD*sin(theta1(t)+theta3(t))*(diff(s1(t), t)))/(LAD*LBC*cos(theta1(t)+theta3(t)-theta2(t))-LAD*LBC*cos(theta1(t)+theta3(t)+theta2(t))), diff(theta3(t), t) = (diff(s1(t), t))*cos(theta1(t))/(L3*sin(theta1(t)+theta3(t))), diff(s2(t), t) = (L1*sin(theta1(t)+theta3(t)-theta2(t))*(diff(s1(t), t))+L1*sin(-theta3(t)-theta2(t)+theta1(t))*(diff(s1(t), t))-LAD*sin(theta1(t)+theta3(t)-theta2(t))*(diff(s1(t), t))-LAD*sin(theta1(t)+theta3(t)+theta2(t))*(diff(s1(t), t)))/(LAD*cos(theta1(t)+theta3(t)-theta2(t))-LAD*cos(theta1(t)+theta3(t)+theta2(t)))]