-1x2 +x +20 = 0
ax2 bx c = 0
a = -1
b = 1
c = 20
Δ = b2-4ac
Δ = ² - (4 * -1 * 20)
Δ = 1 - (4*-20)
Δ = 1 - (-80)
Δ = 81 Δ > 0
-x2+x+20=0
x = -b ∓ √Δ 2a
x' = -1 - √81 2*-1 = -1 - 9.0 -2 = -10.0 -2 = 5.0
x" = -1 + √81 2*-1 = -1 + 9.0 -2 = 8.0 -2 = -4.0
x' = 5.0
x" = -4.0
S = {x ∈ R / x' = 5.0, x" = -4.0 }