I had to teach a class at a job interview for my current position years ago. The department assigned me a topic and I kept thinking, “don’t be work, don’t be work” and it was work!
As a commenter said below, work requires you to a) put together the concepts from Reimann Sums which you likely just learned about and may not have sunk in yet AND b) connect it to a topic that you may not be familiar with.
Like many things in math, once you get it, it doesn’t seem that bad and you wonder why you struggled with it. But getting there takes synthesizing two topics that may be new to you.