arccos(1cos3\pi /4) = arccos(\sqrt{2} /2) = \pi /4
arccos(cos(π-π/4)) = arccos(-cos π/4)) = π - arccos(cos π/4) = π - arccos(√2/2) = π - π/4 = 3π4
arccos(1cos3\pi /4) = arccos(\sqrt{2} /2) = \pi /4
arccos(cos(π-π/4)) = arccos(-cos π/4)) = π - arccos(cos π/4) = π - arccos(√2/2) = π - π/4 = 3π4