Pourquoicos⁡2θ+sin⁡2θ=1\cos^2\theta+\sin^2\theta=1
Le point final du cercle unité est (cos⁡θ;sin⁡θ)(\cos\theta;\sin\theta). Pour les coordonnées sur un cercler=1r=1, Pythagore donne(cos⁡θ)2+(sin⁡θ)2=1.(\cos\theta)^2+(\sin\theta)^2=1.Il est d'usage d'écrire cos⁡2θ+sin⁡2θ=1\cos^2\theta+\sin^2\theta=1 pour tousθ\theta.
The identity does not depend on the quadrant: for everyθ\thetala somme des carrés est11
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