Bonjour
f (x) = ( x - 2 ) ( x + 6 )
f (x) = x² + 6 x - 2 x - 12 = x² + 4 x - 12
( x + 2 )² - 16 = ( x + 2 - 4 ) ( x + 2 + 4 ) = ( x - 2 ) ( x + 6 )
f (x) ≤ 0
( x - 2 )( x + 6 ) ≤ 0
x ≤ 2 ou x ≤ - 6
f (x ) ≤ 20
x² + 4 x - 12 ≤ 20
x² + 4 x ≤ 20 + 12
x² + 4 x ≤ 32
x ( x + 4 ) ≤ 32
x ≤ 2 ou x + 4 ≤ 32 d'où x ≤ 28
f (x) ≤ x² + 4
x² + 4 x - 12 ≤ x² + 4
x² - x² + 4 x ≤ 4 - 12
4 x ≤ - 8
x ≤ - 2
