So the derivative with respect to x of sine of x. By definition, this is going to be the limit as delta x approaches zero of sine of x plus delta x minus sine of x, all of that over delta, all of that over delta x. This is really just the slope of the line between the point x comma sine of x and x plus delta x comma sine of x plus delta x. So how can we evaluate this? Well, we can rewrite sine of x plus delta x using our angle addition formulas that we learned during our trig identities. So this is going to be the same thing as the limit as delta x approaches zero. I'll rewrite this using our trig identity as cosine of x times sine of delta x plus sine of x times cosine of delta x.