1. -8x四次方-16x³y =-8x³(x+2y)
2. （x-y）²+2（y-x)=（x-y）²-2(x-y)=(x-y)(x-y-2)
3. 81a四次方-b四次方=(9a²-b²)(9a²+b²)=(3a+b)(3a-b)(9a²+b²)