bjr
à savoir
cos(a + b) = cos(a) cos(b) − sin(a)sin(b) (A)
cos(a − b) = cos(a) cos(b) + sin(a)sin(b)
sin(a + b) = sin(a) cos(b) + cos(a)sin(b)
sin(a − b) = sin(a) cos(b) − cos(a)sin(b)
je te montre la 1ère
1) on utilise la formule (A)
cos(π/7) x cos (4π/21) - sin(π/7) x sin(4π/21) =
cos(π/7 + 4π/21) =
cos(3π/21 + 4π/21) =
cos(7π/21) = cos(π/3) = 1/2
