bài 12:
`7.(a-5)+8a.(5-a)`
`=7.(a-5)+8a.[-(a-5)]`
`=7.(a-5)-8a.(a-5)`
`=(a-5).(7-8a)`
bài 13:
`5x-5y+ax-ay`
`=5.(x-y)+a.(x-y)`
`=(x-y).(5+a)`
bài 14:
`x^2-xy+x-y`
`=x.(x-y)+(x-y)`
`=(x-y).(x+1)`
bài 15:
`x^2-2xy+8x-16y`
`=(x^2-8x)+(-2xy-16y)`
`=x(x-8) -2y(x-8)`
`=(x-8)(x-2y)`