-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 = -42.0 -2 = 21.0
x" = -20 + √484 2*-1 = -20 + 22.0 -2 = 2.0 -2 = -1.0
x' = 21.0
x" = -1.0
S = {x ∈ R / x' = 21.0, x" = -1.0 }