bjr
1) x 2 6 18
f(x) -1 1 7
f(x) = ax + b
f(2) = -1 => 2a + b = -1 (1)
f(6) = 1 => 6a + b = 1 (2)
on fait (2) - (1)
(6a + b) - (2a + b) = 1 - (-1)
4a = 2
a = 1/2
on remplace dans (1)
2(1/2) + b = -1
1 + b = -1
b = -2
f(x) = (1/2)x -2
f(18) = 9 - 2
f(18) = 7
2)
x 6 12 3
f(x) 8 4 10
f(x) = ax + b
f(6) = 8 d'où 6a + b = 8 (1)
f(12) = 4 d'où 12a + b = 4 (2)
(12a + b) - (6a + b) = 4 - 8
6a = -4
a = -2/3
on remplace dans (1)
6(-2/3) + b = 8
-4 + b = 8
b= 12
f(x) = (-2/3)x + 12
f(x) = 10 équivaut à (-2/3)x + 12 = 10
-2x + 36 = 30
-2x = -6
x = 3
