Illustrative Mathematics
Illustrative Mathematics: F Tf Sum and Difference Angle Formulas
The goal of this task is to have learners derive the addition and subtraction formulas for cosine and tangent, and the subtraction formula for cosine, from the sum formula for sine. Aligns with F-TF.C.9.
Mathigon
Mathigon: Geometry: Triangles and Trigonometry: Pythagoras' Theorem
Pythagoras' Theorem: In any right-angled triangle, the square of the length of the hypotenuse (the side that lies opposite the right angle) is equal to the sum of the squares of the other two sides. (A+B=C)
Mathigon
Mathigon: Geometry: Angles and Polygons: Drawing Triangles
This lesson focuses on the Triangle Inequality, when the sum of the lengths of any two sides of a triangle is greater than the length of the third. When this happens, the three sides cannot form a triangle.
Lawrence Hall of Science
The Math Page: Trigonometry: Trigonometric Identities
This resource clearly and thoroughly explains the many types of trigonometric identities. There are example problems solved, problems for the student to attempt, and answers to the student problems.
Other
Sierra College: Trigonometric Identities
Provides trigonometric identities, which include fundamental identities, even-odd identities, addition formulas, double-angle formulas, subtraction formulas, product-sum formulas, and identities for sin squared and cos squared.
Math Words
Math words.com: Trig Identities
This MathWords.com site lists the equations for all of the trigonometric identities.
Other
Brown Math: Remembering Trig Identities
Faced with a large number of trigonometric identities, students tend to try to memorize them all. This site the emphasis is on understanding how the functions work and on memorizing only about a half dozen of them - all the others work...
Purple Math
Purplemath: The Pythagorean Theorem
Defines the Pythagorean Theorem, and demonstrates how to use this Theorem in connection with right-angled triangles.