[（1/3）a^2-3b^2]/[(1/4)a+3/4b] 上下乘12=(4a^2-36b^2)/(3a+9b)=4(a^2-9b^2)/[3(a+3b)]=4(a+3b)(a-3b)/[3(a+3b)]=4(a-3b)/3[(1/4)x^2-(3/2)xy+2y^2]/[(1/6)x^2-(1/2)xy+(1/3)y^2]上下乘12=(3x^2-18xy+24y^2)/(2x^2...