bonsoir

A(x) = (x - 2)² + 17 = 18

      = x² - 4 x + 4 + 17 = 18

     = x² - 4 x + 21 - 18 = 0

    = x² - 4 x + 3 = 0

Δ = ( - 4)² - 4 ( 1 * 3) = 16 - 12 = 4 = 2²

x₁ = (- ( -4) - 2) / 2 = 2/2 = 1

x ₂ - ( - ( -4) + 2) /2 = 6/2 = 3

S ( 1 : 3)

A(x) = ( x - 2)² + 17 > 26

       = x² - 4 x + 4 + 17 - 26 > 0

       = x² - 4 x - 5 >0

Δ = ( -4) ² - 4 ( 1 * -5) = 16 + 20 = 36 = 6 ²

x ₁ = ( 4 - 6)/2 = - 2/2 = - 1 donc pas solution

x ₂ = ( 4 + 6)/2 = 10/2 = 5

x doit être > 5