à quoi égale (regarder sur l' image) «remarque:b=-2/3 et a=1/4

[2 x 1/4 - (- 2/3)] / [1/4 - 2 x (- 2/3)]
= (2/4 + 2/3) / (1/4 + 4/3)
= (6/12 + 8/12) / (3/12 + 16/12)
= 14/12 / 19/12
= 7/6 / 19/12
= 7/6 x 12/19
= (7 x 12) / (6 x 19)
= (7 x 6 x 2) / (6 x 19)
= (7 x 2) / 19
= 14 / 19