Generalization of Pythagorean Theorem

Adopt from

http://en.wikipedia.org/wiki/Law_of_cosines


In trigonometry, the law of cosines (also known as the cosine formula or cosine rule) is a statement about a general triangle that relates the lengths of its sides to the cosine of one of its angles. Using notation as in Fig. 1, the law of cosines states that

c^2 = a^2 + b^2 - 2ab\cos\gamma\ ,

where γ denotes the angle contained between sides of lengths a and b and opposite the side of length c.

The law of cosines generalizes the Pythagorean theorem, which holds only for right triangles: if the angle γ is a right angle (of measure 90° or π/2 radians), then cos(γ) = 0, and thus the law of cosines reduces to

c^2 = a^2 + b^2 \,

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