arccos(- 1 / 2)
Assume y = arccos(- 1 / 2).
cos y = – 1 / 2 with 0 ≤ y ≤ Π (Cos Theorem) … (1)
Cos (π / 3) = 1 / 2 (Use table of Special Angles)
And also, cos(Π – x) = – cos x.
cos (π – π/3) = – 1 / 2 …(2)
Compare statement 1 with statement 1
y = Π – Π / 3 = 2 Π /3
