原式为：x²-3xy／x²-6xy+9y² =x(x－3y)／(x－3y)² =x／x－3y 再将x=-1 y=3分之2代入，有原式=x／x－3y =﹣1／（﹣1）－3×2／3 =1／3
(X^2-6XY+9Y^2)/(X^2-3XY)=(X-3Y)^2/x(x-3y)=(x-3y)/x=1-3y/x1-3y/x=1-(3*2/3)/(-1)=1+2=3
