Pouvez vous m’aider à faire l’exo 1 svp car je ne comprend pas

EX1
cos(5π/6) = cos(π - π/6) = - cos(π/6) = - √3/2
sin(5π/4) = sin(π + π/4) = - sin(π/4) = - √2/2
cos (3π) = cos(π + 2π) = - cos(2π) = - 1
3 π = 2 π + π