law of cosines

Definition of law of cosines 

1 : a law in trigonometry: the square of a side of a plane triangle equals the sum of the squares of the remaining sides minus twice the product of those sides and the cosine of the angle between them

2 : a law in trigonometry: the cosine of an arc of a spherical triangle equals the product of the cosines of the remaining arcs plus the product of the sines of those arcs and the cosine of the angle between them

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