-4x2 +21x -20 = 0
ax2 bx c = 0
a = -4
b = 21
c = -20
Δ = b2-4ac
Δ = 21² - (4 * -4 * -20)
Δ = 441 - (4*80)
Δ = 441 - (320)
Δ = 121 Δ > 0
-4x2+21x-20=0
x = -b ∓ √Δ 2a
x' = -21 - √121 2*-4 = -21 - 11.0 -8 = -32.0 -8 = 4.0
x" = -21 + √121 2*-4 = -21 + 11.0 -8 = -10.0 -8 = 1.25
x' = 64.0
x" = 20.0
S = {x ∈ R / x' = 64.0, x" = 20.0 }