bonsoir
x ⁴ - 11 x² + 28 = 0
X² - 11 X + 28 = 0
Δ = ( - 11)² - 4 ( 1 * 28) = 121 - 112 = 9 = 3²
X₁ =( - ( - 11) - 3 ) / 2 = 8/2 = 4
X² = ( - ( -11) + 3) /2 = 14/2 = 7
x² = 4 donc x = √ 4 ou - √4 = 2 ou - 2
x² = 7 donc x = √7 ou - √7
S = ( - 2 , 2 ; - √7 ; √7)
( 2 x - 4) / (x - 7) = 0
x ≠ 7
2 x - 4 = 0 ⇔ x = 2
x² - 9 x ≥ 90
x² - 9 x - 90 ≥0
Δ = ( - 9)² - 4 ( 1 * -90)
Δ = 81 + 360 = 441 = 21 ²
x₁ = - ( - 9) - 21) /2 = - 12/2 = - 6 donc pas de solution
x² = ( 9 + 21)/2 = 30/2 = 15
S [ 15 ; + ∞ [
