bonjour
f ( 5) = - 1 et f ( 15 ) = - 9
( - 9 + 1 ) / ( 15 - 5 ) = - 8 /10 = - 4 /5
donc ax = - 4 x /5
f ( 5) = - 1
5 * - 4/5 + b = - 1
⇔ b = - 1 + 20/5 = - 1 + 4 = 3
f (x) = - 4 x /5 + 3
d'où f ( 15 ) = 15 * - 4/5 + 3 = - 60/5 + 3 = - 9
f ( 25) = 25 * - 4/5 + 3 = - 20/5 + 3 = - 17
