Hi Amir,

most likely you first executed

b := a'[t] Sin[a[t]]

and then the correct definition

b[a_, t_] := a'[t] Sin[a[t]]

Do a clean start or at least execute

ClearAll[b]

then it will work.

Regards Henrik

Thank you Henrik now it works correctly

Dear Henrik most recently i encountered with another problem in mathematica NDSolve i had defined some equations "lagrangeo1" ,"lagrangei1", and "lagrangei2" that are differential equations. but when started to solve:

lagrangesolve = NDSolve[{lagrangei1 == 0, lagrangei2 == 0, lagrangeo1 == 0,o1[0] == -36,i1[0] ==18,i2[0] == 18, o1'[0] == 0.5, i1'[0] == 0,i2'[0] == -0.75}, {o1, i1, i2}, {t, 0, 100}]

(o1,i1,i2 are my variables) it said :

NDSolve::ndsdtc: The time constraint of 1.` seconds was exceeded trying to solve for derivatives, so the system will be treated as a system of differential-algebraic equations. You can use Method->{"EquationSimplification"->"Solve"} to have the system solved as ordinary differential equations. >> NDSolve::njnum: The Jacobian is not a matrix of numbers at ... ... General::stop: Further output of NDSolve::njnum will be suppressed during this calculation. >> NDSolve::icfail: Unable to find initial conditions that satisfy the residual function within specified tolerances. Try giving initial conditions for both values and derivatives of the functions. >>

will be glad if you could help me again. regards

Dears most recently i encountered with another problem in mathematica NDSolve i had defined some equations "lagrangeo1" ,"lagrangei1", and "lagrangei2" that are differential equations. but when started to solve:

lagrangesolve = NDSolve[{lagrangei1 == 0, lagrangei2 == 0, lagrangeo1 == 0,o1[0] == -36,i1[0] ==18,i2[0] == 18, o1'[0] == 0.5, i1'[0] == 0,i2'[0] == -0.75}, {o1, i1, i2}, {t, 0, 100}]

(o1,i1,i2 are my variables) it said :

NDSolve::ndsdtc: The time constraint of 1.` seconds was exceeded trying to solve for derivatives, so the system will be treated as a system of differential-algebraic equations. You can use Method->{"EquationSimplification"->"Solve"} to have the system solved as ordinary differential equations. >> NDSolve::njnum: The Jacobian is not a matrix of numbers at ... ... General::stop: Further output of NDSolve::njnum will be suppressed during this calculation. >> NDSolve::icfail: Unable to find initial conditions that satisfy the residual function within specified tolerances. Try giving initial conditions for both values and derivatives of the functions. >>

will be glad if anyone could help me. regards