-1x2 -4x +21 = 0
ax2 bx c = 0
a = -1
b = -4
c = 21
Δ = b2-4ac
Δ = -4² - (4 * -1 * 21)
Δ = 16 - (4*-21)
Δ = 16 - (-84)
Δ = 100 Δ > 0
-x2-4x+21=0
x = -b ∓ √Δ 2a
x' = 4 - √100 2*-1 = 4 - 10.0 -2 = -6.0 -2 = 3.0
x" = 4 + √100 2*-1 = 4 + 10.0 -2 = 14.0 -2 = -7.0
x' = 3.0
x" = -7.0
S = {x ∈ R / x' = 3.0, x" = -7.0 }