令1/3x²-1/2x-1=0
x=[1/2±√(1/4+4/3)]/(2/3)
=[1/2±√19/12]/(2/3)
=（6±√19)/8
∴1/3x²-1/2x-1=1/3[x-(6+√19)/8][x-(6-√19)/8]