-15x2 +6x +21 = 0
ax2 bx c = 0
a = -15
b = 6
c = 21
Δ = b2-4ac
Δ = 6² - (4 * -15 * 21)
Δ = 36 - (4*-315)
Δ = 36 - (-1260)
Δ = 1296 Δ > 0
-15x2+6x+21=0
x = -b ∓ √Δ 2a
x' = -6 - √1296 2*-15 = -6 - 36.0 -30 = -42.0 -30 = 1.4
x" = -6 + √1296 2*-15 = -6 + 36.0 -30 = 30.0 -30 = -1.0
x' = 315.0
x" = -225.0
S = {x ∈ R / x' = 315.0, x" = -225.0 }