a) 5-1=4
2×4-1=7
2×7-1=13
13²-1=168
b) 0-1=-1
-1×2-1=-3
-3×2-1=-7
(-7)²-1=48
c) n-1 = n-1
(n-1)×2-1=2n-2-1=2n-3
(2n-3)×2-1=4n-6-1=4n-7
(4n-7)²-1=(4n)²-2(4n)(-7)+7²-1=16n²-56n+49-1=16n²-56n+48
or : 16n²-56n+48 = 8 × (2n²-7n+6)
quelle que soit la valeur de n, 8 × (2n²-7n+6) sera toujours un multiple
de 8
