-12x三次方y四次方除（-3x平方y三次方）乘（-三分之一xy）=(-4/3)x^(3-2+1)y^(4-3+1)=-4x²y²/3
-12x^3y^4÷(-3x^2y^3)*(-xy/3)=(-12)÷(-3)*(-1/3)*x^(3-2+1)y^(4-3+1)=-4/3*x^2y^2
-12x^3y^4/(-3x^2)y^3*(-xy/3)=-4x^2y^2/3
-3/4*(xy)^2
