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10π/20 = π/2

cos(π/2) = 0    et    sin(π/2) = 1  (voir cercle trigonométrique)

a) 1/2 cos (10π/20) = 1/2 cos (π /2) = 1/2 x 0 = 0

b) 1/2 sin (10π/20) = 1/2 sin(π/2) = 1/2 x 1 = 1/2  

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