Varsity Tutors
Varsity Tutors: Hotmath: Practice Problems: Sum and Difference Formulas
Twenty problems are presented for practice working with sum and difference formulas for sine, cosine, and tangent.
CK-12 Foundation
Ck 12: Trigonometry: Tangent Sum and Difference Formulas
[Free Registration/Login may be required to access all resource tools.] Here you'll learn to rewrite tangent functions with addition or subtraction in their arguments in a more easily solvable form.