-4x2 +5x +21 = 0
ax2 bx c = 0
a = -4
b = 5
c = 21
Δ = b2-4ac
Δ = 5² - (4 * -4 * 21)
Δ = 25 - (4*-84)
Δ = 25 - (-336)
Δ = 361 Δ > 0
-4x2+5x+21=0
x = -b ∓ √Δ 2a
x' = -5 - √361 2*-4 = -5 - 19.0 -8 = -24.0 -8 = 3.0
x" = -5 + √361 2*-4 = -5 + 19.0 -8 = 14.0 -8 = -1.75
x' = 48.0
x" = -28.0
S = {x ∈ R / x' = 48.0, x" = -28.0 }