nilai dari cos 5r/6 adalah

5/6 π = 5/6 x 180° = 150°
cos 5/6 π = cos 150° = -cos (180 - 30)° = -cos 30° = -1/2 √3
sin 5/6 π = sin 150° = sin (180 - 30)° = sin 30° = 1/2
tan 5/6 π = tan 150° = sin 30°/-cos 30° = 1/2 / -(1/2 √3) = -1/√3 = -1/3 √3

Semangat!