Hi, I have got this differential equation: y''=x*sin(y) I am trying to use Mathematica 9 to solve this equation, so I write the following expression: A = First[ y /. NDSolve[{y''[x] == x*Sin[y[x]], y[0] == 1, y'[0] == 1}, y, {x, 0, 19}]]

But Mathematica always answers with something like that:

"NDSolve::deqn: Equation or list of equations expected instead of True in the first argument {(y^[Prime][Prime])[x]==x Sin[y[x]],y[0]==1,True}. >>" And "ReplaceAll::reps: {NDSolve[{(y^[Prime][Prime])[x]==x Sin[y[x]],y[0]==1,True},y,{x,0,19}]} is neither a list of replacement rules nor a valid dispatch table, and so cannot be used for replacing. >>"

But a friend of mine has tried to input exactly what I wrote, and she got a normal result. She has Mathematica 10.

By answering a question, how many maximum and minimum has the function, the answer should be 8. And my question is: Is there any problem with Mathematica or can you see if I write something wrong?

Thanks to anyone who has tried solving this