5x2 +21x +18 = 0
ax2 bx c = 0
a = 5
b = 21
c = 18
Δ = b2-4ac
Δ = 21² - (4 * 5 * 18)
Δ = 441 - (4*90)
Δ = 441 - (360)
Δ = 81 Δ > 0
5x2+21x+18=0
x = -b ∓ √Δ 2a
x' = -21 - √81 2*5 = -21 - 9.0 10 = -30.0 10 = -3.0
x" = -21 + √81 2*5 = -21 + 9.0 10 = -12.0 10 = -1.2
x' = -75.0
x" = -30.0
S = {x ∈ R / x' = -75.0, x" = -30.0 }