Illustrative Mathematics
Equivalent Expressions
Here is a straight-forward problem of multiplying two binomials with a twist. It is up to algebra learners to decide how to turn this product of sums into a sum of products. However, it is not the quadratic that is the answer; it is the...
Wolfram Research
Wolfram Math World: Sum
This page gives a complete discussion of the advanced applications of the summation operation in mathematics. Provides information about sigma, or summation notation, a manner of writing sums and how to manipulate them. The site also...