-1x2 -9x -20 = 0
ax2 bx c = 0
a = -1
b = -9
c = -20
Δ = b2-4ac
Δ = -9² - (4 * -1 * -20)
Δ = 81 - (4*20)
Δ = 81 - (80)
Δ = 1 Δ > 0
-x2-9x-20=0
x = -b ∓ √Δ 2a
x' = 9 - √1 2*-1 = 9 - 1.0 -2 = 8.0 -2 = -4.0
x" = 9 + √1 2*-1 = 9 + 1.0 -2 = 10.0 -2 = -5.0
x' = -4.0
x" = -5.0
S = {x ∈ R / x' = -4.0, x" = -5.0 }