the cosine rule is derived from the division of the adjacent side & hypotenuse
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The answer depends on the information that you have: it could be the sine rule or the cosine rule.
No. Sine rule (and cosine rule) apply to all triangles in Euclidean space (plane geometry). A simplification occurs when there is a right angle because the sine of the right angle is 1 and the cosine is 0. Thus you get Pythagoras theorem for right triangles.
It follows from the cyclical symmetry of the cosine rule.
This is known as the Cosine Rule.
Inverse of Cosine is 'ArcCos' or Cos^(-1) The reciprocal of Cosine is !/ Cosine = Secant.