-4x2 -8x +21 = 0
ax2 bx c = 0
a = -4
b = -8
c = 21
Δ = b2-4ac
Δ = -8² - (4 * -4 * 21)
Δ = 64 - (4*-84)
Δ = 64 - (-336)
Δ = 400 Δ > 0
-4x2-8x+21=0
x = -b ∓ √Δ 2a
x' = 8 - √400 2*-4 = 8 - 20.0 -8 = -12.0 -8 = 1.5
x" = 8 + √400 2*-4 = 8 + 20.0 -8 = 28.0 -8 = -3.5
x' = 24.0
x" = -56.0
S = {x ∈ R / x' = 24.0, x" = -56.0 }