x³ - 3x² - 9 = 0用盛金公式解a=1,b= -3,c=0,d= -9A=b²-3ac = 9B=bc-9ad=81C=c²-3bd=-81Δ=B²-4AC = 9477 > 0方程只有一个实数根x = [ -b - (³√y₁ +³√y₂)]/(3a)其...