bonsoir
f ( 7 ) = 2.8 et f( 10 ) = 3
( 3 - 2.8 ) / ( 10 - 7 ) = 0.2 / 3 = 2 /30 = 1 /15
ax = x /15
f ( 10 ) = 3
10*1/15 + b = 3
b = 3 - 10/15 = 45/15 - 10 /15 = 35/15 = 7 /3
f(x) = x /15 + 7/3
d'où f ( 7) = 7 * 1/15 + 7/3 = 7/15 + 35/15 = 42/15 = 2.8
f ( 15 ) = 5
f (x) = 5x /15 = x / 3
f ( - 3 ) = - 3 /3 = - 1
f ( 2/5) = 2/5 x 1/3 = 2 /15
x /3 = 2
x = 6
x /2 = - 9
x = - 18
