-3x/(x²-3x)=t
tx^2-3xt+3x=0
5x^2+3x(1-t)=0
x(5x+3-3t)=0
x=0(舍去）,或5x+3-3t=0
t=(5x/3)+1
当x为3的倍数时,t即-3x/x²-3x为整式.