-9x2 +20x +21 = 0
ax2 bx c = 0
a = -9
b = 20
c = 21
Δ = b2-4ac
Δ = 20² - (4 * -9 * 21)
Δ = 400 - (4*-189)
Δ = 400 - (-756)
Δ = 1156 Δ > 0
-9x2+20x+21=0
x = -b ∓ √Δ 2a
x' = -20 - √1156 2*-9 = -20 - 34.0 -18 = -54.0 -18 = 3.0
x" = -20 + √1156 2*-9 = -20 + 34.0 -18 = 14.0 -18 = -0.777777777778
x' = 243.0
x" = -63.0
S = {x ∈ R / x' = 243.0, x" = -63.0 }