-4x2 +21x -5 = 0
ax2 bx c = 0
a = -4
b = 21
c = -5
Δ = b2-4ac
Δ = 21² - (4 * -4 * -5)
Δ = 441 - (4*20)
Δ = 441 - (80)
Δ = 361 Δ > 0
-4x2+21x-5=0
x = -b ∓ √Δ 2a
x' = -21 - √361 2*-4 = -21 - 19.0 -8 = -40.0 -8 = 5.0
x" = -21 + √361 2*-4 = -21 + 19.0 -8 = -2.0 -8 = 0.25
x' = 80.0
x" = 4.0
S = {x ∈ R / x' = 80.0, x" = 4.0 }