Over here, the derivative of cosine of x looks like it is zero. And negative sine of x is indeed zero. So it actually turns out that it is the case, that the derivative of cosine of x is negative sine of x. So these are really good to know. These are kind of fundamental trigonometric derivatives to know, we'll be able to derive other things for them. And hopefully this video gives you a good intuitive sense of why this is true. And in future videos, we will prove it rigorously.