解:原方程化为4x²-31x-45=0 (4x+5)(x-9)=0 4x+5=0或x-9=0 x1=-0.8,x2=9
4x²-45=31x4x²-31-45=0(4x+5)(x-9)=0x=-5/4,x=9
十字相乘法:(4x+5)(x-9)=0 可解得x=9或x=-1.25
x=9