**Title:** Law of Cosines

**Description:**
In this Illuminations lesson, students use right triangle trigonometry and the Pythagorean theorem to develop the law of cosines. Included is a link to an online activity sheet.

**Standard(s): **

[MA2013] GEO (9-12) 22: (+) Prove the Law of Sines and the Law of Cosines and use them to solve problems. [G-SRT10]

[MA2013] GEO (9-12) 23: (+) Understand and apply the Law of Sines and the Law of Cosines to find unknown measurements in right and non-right triangles (e.g., surveying problems, resultant forces).
[G-SRT11]

[MA2013] AM1 (9-12) 17: (+) Prove the Law of Sines and the Law of Cosines and use them to solve problems. Understand Law of Sines = 2*r*, where *r* is the radius of the circumscribed circle of the triangle. Apply the Law of Tangents. [G-SRT10] (Alabama)