-1x2 +18x +19 = 0
ax2 bx c = 0
a = -1
b = 18
c = 19
Δ = b2-4ac
Δ = 18² - (4 * -1 * 19)
Δ = 324 - (4*-19)
Δ = 324 - (-76)
Δ = 400 Δ > 0
-x2+18x+19=0
x = -b ∓ √Δ 2a
x' = -18 - √400 2*-1 = -18 - 20.0 -2 = -38.0 -2 = 19.0
x" = -18 + √400 2*-1 = -18 + 20.0 -2 = 2.0 -2 = -1.0
x' = 19.0
x" = -1.0
S = {x ∈ R / x' = 19.0, x" = -1.0 }