Pythagorean theorem is used only for the right angled triangles.
For ΔABC,
Cosine rule is
[tex] a^2=b^2 + c^2-2bc$ cosA $ [/tex]
[tex] b^2=a^2 + c^2-2ac$ cosB $ [/tex]
[tex] c^2=a^2 + b^2-2ab$ cosC $ [/tex]
Here we have used one of these formulae
Hence definition of cosine or Cosine rule is the right answer
Option 2) is the right answer
