-1x2 +19x +20 = 0
ax2 bx c = 0
a = -1
b = 19
c = 20
Δ = b2-4ac
Δ = 19² - (4 * -1 * 20)
Δ = 361 - (4*-20)
Δ = 361 - (-80)
Δ = 441 Δ > 0
-x2+19x+20=0
x = -b ∓ √Δ 2a
x' = -19 - √441 2*-1 = -19 - 21.0 -2 = -40.0 -2 = 20.0
x" = -19 + √441 2*-1 = -19 + 21.0 -2 = 2.0 -2 = -1.0
x' = 20.0
x" = -1.0
S = {x ∈ R / x' = 20.0, x" = -1.0 }