（x平方+3x）-（x平方+3x）分之20=8

设x²+3x=y,则原方程可化为y-20/y=8y²-20=8yy²-8y-20=0(y-10)(y+2)=0y1=10,y2=-2所以x²+3x=10,x²+3x=-2x²+3x-10=0,x²+3x+2=0(x+5)(x-2)=0,(x+1)(x+2)=0x1=-5,x2=2,x3=-1,x4=-2...