C(x) = ( x + 1) ( 2 - x ) - 2 ( x + 1) ( 2 x + 3)
C(x) = 2 x - x² + 2 - x - ( 2 x + 2) ( 2 x + 3)
C(x) = 2 x - x² + 2 - x - ( 4 x² + 6 x + 4 x + 6)
C(x) = - x² + x + 2 - 4 x² - 6 x - 4 x - 6
C(x) = - 5 x² - 9 x - 4
C(x) = ( x + 1) ( 2 - x - 4 x - 6)
C(x) = ( x + 1) ( - 5 x - 4 )
C(x) pour x = - 1
C(x) = ( - 1 + 1) ( 5 - 4) = 0
C(x) = 0
(x + 1) ( - 5 x - 4) = 0
soit x + 1 = 0 et x = - 1
soit - 5 x - 4 = 0 donc - 5 x = 4 et x = - 4/5