Law of Cosines

The Law of Cosines is used only on non-right triangles.

The formula for the law of cosines is:

leg^2 = adjacent leg^2 + other adj leg^2 - 2(adj leg)(other adj leg)cos(angle between)

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Example:

Triangle ABC, where a = 7, c = 3, and B = 130 degrees.

b^2 = 3^2 +7^2 - 2(3)(7)cos130 degrees

b ~ 9.219

7^2 = 3^2 + 9.219^2 - 2(3)(9.219)cosA

-2(3)(9.219)cosA = 7^2 - 3^2 - 9.219^2

cosA = (7^2 - 3^2 - 9.219^2)/(-2(3)(9.219))

cosA = .813

There you go.