Bonjour, j'ai un exercice et je voudrais savoir si mes calculs sont bon.
1/ Sachant que cos(pi/5) = ((racine5)+1)/4 , Calculer Sin(pi/5)
2/ En déduire les cos et sin de -pi/5, 4pi/5 et 3pi/10
1/cos²(pi/5) + sin²(x) = 1
(((racine5)+1)/4) + sin²(x) = 1
Sin²(x) = 1- (((racine5)+1)/4) = (4-racine5+1)/4 = (5-racine5)/4
Sin(x) = Racine( (5-racine5)/8)
Cos(x) = Sin((pi/2)-x)
= Sin( (pi/2) - (pi/5)
= Sin (3pi/10)
2/ Cos(-pi/5) = Cos(pi/5) = Sin(3pi/10) = ((racine5)+1)/4
Sin(-pi/5) = -Sin(pi/5) = - Racine( (5-racine5)/8)
Cos(4pi/5) = Sin((pi/2)-(4pi/5) = Sin(-3pi/10) =-((racine5)+1)/4)
Sin(4pi/5) = Sin(pi-(4pi/5)) = Sin(pi/5) = Racine( (5-racine5)/8)
Cos(3pi/10) = Sin((pi/2)-(3pi/10) = Sin(pi/5) = Racine( (5-racine5)/8)
Sin(3pi/10) = Cos((pi/2)-(3pi/10) = Cos(pi/5) = ((racine5)+1)/4
Merci d'avance !
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