-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 = -14.0 -2 = 7.0
x" = -4 + √100 2*-1 = -4 + 10.0 -2 = 6.0 -2 = -3.0
x' = 7.0
x" = -3.0
S = {x ∈ R / x' = 7.0, x" = -3.0 }