Khan Academy
Khan Academy: Understand Subtraction as Adding the Opposite
Practice rewriting subtraction problems as equivalent addition problems. For example, 2-9=2+(-9). Students receive immediate feedback and have the opportunity to try questions repeatedly, watch a video or receive hints.
CK-12 Foundation
Ck 12: Trigonometry: Cosine Sum and Difference Formulas
[Free Registration/Login may be required to access all resource tools.] Examples, videos, and practice problems covering cosine of a sum or difference related to a set of cosine and sine functions. Here you'll learn to rewrite cosine...
Discovery Education
Discovery Education: Web Math: Factor Polynomial by Rewriting It as the Difference Between Two Squares
This WebMath site includes an interactive problem solver. Use it to factor a polynomial that is the difference between two perfect squares. The website will bring back a full explanation behind the answer.