Massachusetts Institute of Technology: Srini Devadas and Eric Lehman's "Counting III"
Read Section 2, which defines the binomial expansion expressed in the binomial theorem, that is, the expansion of (a + b)n. The binomial expansion is very important because it is used to make approximations in many domains, including the sciences, engineering, weather forecasting, economics, and polling.
The expansion of (a + b)n is a polynomial whose coefficients are the integers in the nth row of Pascal's Triangle.