bonjour
f (x) = - ( x - 80)² + 625
f (x) = - ( x² - 160 x + 6 400 ) + 625
f ( x) = - x² + 160 x - 6400 + 625
f (x) = - x² + 160 x - 5 775
Δ = 160 ² - 4 ( - 1 * - 5 775 ) = 2 500 = ( 50 )²
x 1 = ( - 160 - 50 ) / - 2 = 210/2 = 105
x 2 = ( - 160 + 50 ) / - 2 = 110/2 = 55