Série 2 : exo 6e

[1, 4, 0,  7]
[2, 7, 0, 10]

L_2 <- L_2 - 2 * L_1

[1,  4, 0,  7]
[0, -1, 0, -4]

L_2 <- -1 * L_2

[1, 4, 0, 7]
[0, 1, 0, 4]

L_1 <- L_1 - 4 * L_2

[1, 0, 0, -9]
[0, 1, 0,  4]