Here's the question you clicked on:
anyward
Some of the questions in the problem sets refer to equations (i.e. "prove using (2)") but how do you know which equation is which number?
Can you provide a link to the problem set ?
Those references pertain to materials made available for a different calculus course, available here: http://ocw.mit.edu/courses/mathematics/18-01-single-variable-calculus-fall-2006/readings/course-reader/ Click on the PDF link for the relevant topic, such as approximation (if that's what you're dealing with). Equation numbers appear in those materials.