1、解:(1)5xy/20x²=y/4x;
(2)-3mn/6mn²=1/2n;
(3)(a-b)²/(b-a)³
=(b-a)/(b-a)³
=1/b-a
(4)6a(b-1)/8ac(1-b)
=6a(b-1)/-8ac(b-1)
=-3/4c
2、解:(1)x²-9/(x-3)²
=(x-3)(x+3)/(x-3)²
=x+3/x-3
(2)x²y+xy²/x²-y²
=xy(x+y)/(x-y)(x+y)
=xy/x-y
3、解:(1)错误,-a+b/a-b=-(a-b)/a-b=-1;
(2)正确;
(3)错误,m² -n²/m-n=(m-n) (m+n)/m-n