La proportionnalité

y proportionnel à x lorsque $y = a \times x$ avec a = constante appelée coefficient de proportionnalité.

Ici, on a $y = a \times x$.

Le graphique :

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Si en traçant y en fonction de x, on trouve une droite qui passe par l'origine,on peut dire que les deux grandeurs sont proportionnelles.

Pour trouver le coefficient de proportionnalité, on prend un pointsitué sur la droite(par forcément un point du tableau de mesures !) et on calcule $y \frac x$ pour ce point.

U proportionnel à I pour une résistance : $U = R \times I$