a/a-3-a+6/a平方-3a+3/a,
=a/(a-3)-(a+6)/a(a-3)+3/a
=(a²-3a-a-6+3a-9)/a(a-3)
=(a²-a-15)/a(a-3)
x平方/x-1-x-1,
=(x²-x²+2x-1)/(x-1)
=(2x-1)/(x-1)
81x四次方-16y四次方,
=(9x²-4y²)(9x²+4x²)
=(3x-2y)(3x+2y)(9x²+4y²)
（x平方+4）平方-16x平方
=(x²+4-4x)(x²+4+4x)
=(x-2)²(x+2)²