Bonjour !
x² + 1.8x - 16=0
<=> x² + 2 * 0.9 * x + 0.9² - 0.9² - 16 = 0
<=> (x + 0.9)² - 0.81 - 16 = 0
<=> (x + 0.9)² - 16.81 = 0
<=> (x + 0.9)² - 4.1² = 0 (3e identité remarquable)
<=> (x + 0.9 + 4.1)(x + 0.9 - 4.1) = 0
<=> (x + 5)(x - 3.2) = 0 EPN
x + 5 = 0 ou x - 3.2 = 0
x = -5 ou x = 3.2
S = {-5 ; 3.2}
Voilà !
