youcubed: Sums Investigation
youcubed (Stanford University)

This is another investigation where students can practice addition and think about combinatorics – a nice way of organizing counting.

youcubed: Ice Cream Scoop
youcubed (Stanford University)

This is a really nice task as it is open to everyone, can be solved in different ways and can also extend to work in combinatorics – a nice way of organizing counting.

Khan Academy: Pre-Calculus
Khan Academy

The Precalculus course, often taught in the 12th grade, covers Polynomials; Complex Numbers; Composite Functions; Trigonometric Functions; Vectors; Series, Conic Sections, Probability and Combinatorics.