Bonjour
Rappel : Pour tout réel x, cos²(x) + sin²(x) = 1
sin(a) = 3/5
cos²(a) + sin²(a) = 1
cos²(a) + (3/5)² = 1
cos²(a) = 1 - 9/25 = 16/25 = (4/5)²
Comme ici cos(a) ≥ 0
cos(a) = 4/5
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Bonjour
Rappel : Pour tout réel x, cos²(x) + sin²(x) = 1
sin(a) = 3/5
cos²(a) + sin²(a) = 1
cos²(a) + (3/5)² = 1
cos²(a) = 1 - 9/25 = 16/25 = (4/5)²
Comme ici cos(a) ≥ 0
cos(a) = 4/5