Pause the video and see if you can figure this out. Well, with both sine of x and cosine of x, they're defined for all real numbers, so their domain is all real numbers. You can put any real number in here for x and it will give you an output. It is defined. And they are also continuous over their entire domain. In fact, all of the trigonometric functions are continuous over their entire domain. And so for sine of x, because it's continuous and it's defined at sine of pi, we would say that this is the same thing as sine of pi, and sine of pi, you might already know, is equal to zero.