I needed to solve following differential equation:
{y''[x] == -Sin[y[x]], y[0] == 1, y'[0] == 0}. <br/>
However calling
NDSolve[{y''[x] == -Sin[y[x]], y[0] == 1, y'[0] == 0}, y, {x, 0, 30}]
produced an error
NDSolve::deqn: Equation or list of equations expected instead of True in the first argument {(y^\[Prime]\[Prime])[x]==-Sin[y[x]],y[0]==1,True}.
After creating a new Notebook and trying example from the documentation of NDSolve (Namely this one:
s = NDSolve[{y''[x] + Sin[y[x]] y[x] == 0, y[0] == 1, y'[0] == 0},
y, {x, 0, 30}]
I get as an output still:
NDSolve::deqn: Equation or list of equations expected instead of True in the first argument {Sin[y[x]] y[x]+(y^\[Prime]\[Prime])[x]==0,y[0]==1,True}.
I tried ClearAll[y, x] but it didn't seem to work..
My Mathematica Version is 12.1, installed on Windows Pro 64-bit, Version 2004, OS build 19041.572
Thanks a lot for your feedback!