Find the 999th derivative of cos x.

Question

I got sin x.

Answer 1

1st derivative: -sinx
2nd derivative: -cosx
3rd derivative: sinx
4th derivative: cosx
5th derivative: -sinx
..
14th derivative:
etc.
notice we are in a cyclical loop of 4 results,
notice 999 leaves a remainder of 3 when divided by 4, so the result is the same as
the 3rd derivative, namely

sinx