bonjour
f ( 5 ) = - 1 et f ( 15 ) = - 9
( - 9 + 1 ) : ( 15 - 5 ) = - 8 / 10 = - 4 /5
ax = - 4 x / 5
f ( 5) = - 1
5 * - 4 /5 + b = - 1
- 20 /5 + b = - 1
b = - 1 + 4 = 3
f (x) = - 4 x /5 + 3
d'où f ( 25 ) = - 4/5 * 25 + 3 = - 100 /5 + 3 = - 17
