-4x2 -20x -9 = 0
ax2 bx c = 0
a = -4
b = -20
c = -9
Δ = b2-4ac
Δ = -20² - (4 * -4 * -9)
Δ = 400 - (4*36)
Δ = 400 - (144)
Δ = 256 Δ > 0
-4x2-20x-9=0
x = -b ∓ √Δ 2a
x' = 20 - √256 2*-4 = 20 - 16.0 -8 = 4.0 -8 = -0.5
x" = 20 + √256 2*-4 = 20 + 16.0 -8 = 36.0 -8 = -4.5
x' = -8.0
x" = -72.0
S = {x ∈ R / x' = -8.0, x" = -72.0 }