-9x2 +5x +4 = 0
ax2 bx c = 0
a = -9
b = 5
c = 4
Δ = b2-4ac
Δ = 5² - (4 * -9 * 4)
Δ = 25 - (4*-36)
Δ = 25 - (-144)
Δ = 169 Δ > 0
-9x2+5x+4=0
x = -b ∓ √Δ 2a
x' = -5 - √169 2*-9 = -5 - 13.0 -18 = -18.0 -18 = 1.0
x" = -5 + √169 2*-9 = -5 + 13.0 -18 = 8.0 -18 = -0.444444444444
x' = 81.0
x" = -36.0
S = {x ∈ R / x' = 81.0, x" = -36.0 }