Bonjour,

exercice 1 :

89π/6 = (84π + 5π )/6 = = 14π + 5π/6

77π/4 = (80π - 3π/4 ) = 20π - 3π/4

Voir pièce jointe pour le placement

Cos (89π/6) = cos (5π/6) = - cos (π/6) = √3/2

Sin (89π/6) = sin (5π/6) = cos (π/6) = 1/2

Cos (77π/4) = cos( -3π/4) = - cos (π/4) = -√2/2

Sin (77π/4) = sin (-3π/4) = sin (-π/4) = - sin (π/4) = -√2/2

exercice 2 :

∆ = b^2 - 4ac = 1 - 4 × 21 × (- 2 ) = 169>0

Donc deux racines :

x1 = (-b - √∆)/2a = (1 - √169)/(2 × 21) = -12/42 = -2/7

x2 = (-b + √∆)/2a = (1 + √169)/(2×21) = 14/42 = 2/6 = 1/3

Édit : On voit la moitié de la pièce jointe, les points sont à gauche sur le cercle trigonométrique