# NDSolve Example doesn't work

Posted 6 months ago
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 I needed to solve following differential equation: {y''[x] == -Sin[y[x]], y[0] == 1, y'[0] == 0}.
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 19041.572Thanks a lot for your feedback!
 Hi Pavel,Unable to reproduce sol = NDSolve[{y''[x] == -Sin[y[x]], y[0] == 1, y'[0] == 0}, y, {x, 0, 30}] Plot[y[x] /. First@sol, {x, 0, 30}] I am running "12.1.1 for Mac OS X x86 (64-bit) (June 19, 2020)", but I doubt the problem has to do with version or platform.Try restarting the kernel.