If y''=-y prove that there is a solution

[phawksbball24]

If y" = -y prove that there is a solution. I am stuck at the start with this problem, I know what I would do with the first derivative but I am not sure what to do when it is the second derivative, any help you could give me would be greatly appreciated, thanks.

 

[pka]

\(\displaystyle y = \sin(x) ??\)