①.8a²b²c²÷12a²bc ②.5mn(a-b)/-15m³（b-a) ③..6-2x / 9+x²-6x ④.7a²（a+b-c) / 28ab(c-a-b）⑤.

8a²b²c²÷12a²bc =2bc/3(分子分母同除4a²bc)
5mn(a-b)/-15m³（b-a) =n/3m(分子分母同除5m(a-b))
6-2x / 9+x²-6x=2(3-x)/(x-3)^2=2/(3-x)
7a²（a+b-c) / 28ab(c-a-b) =-a/4b(分子分母同除7a（a+b-c) )
3x²-3xy / x²-2xy+y²=3x(x-y)/(x-y)^2=3x/(x-y)
4b²-a² / 4b²-4ab+a²=(2b+a)(2b-a)/(2b-a)^2=(2b+a)/(2b-a)
3x²-18x+27 / 2x²-18=3(x^2-6x+9)/2(x^2-9)=3(x-3)^2/2(x+3)(x-3)=3(x-3)/2(x+3)