-1x2 -20x +21 = 0
ax2 bx c = 0
a = -1
b = -20
c = 21
Δ = b2-4ac
Δ = -20² - (4 * -1 * 21)
Δ = 400 - (4*-21)
Δ = 400 - (-84)
Δ = 484 Δ > 0
-x2-20x+21=0
x = -b ∓ √Δ 2a
x' = 20 - √484 2*-1 = 20 - 22.0 -2 = -2.0 -2 = 1.0
x" = 20 + √484 2*-1 = 20 + 22.0 -2 = 42.0 -2 = -21.0
x' = 1.0
x" = -21.0
S = {x ∈ R / x' = 1.0, x" = -21.0 }