解(1)∵最简公分母是18x3y3,
∴
1
3xy3=
6x2
3xy3•6x2=
6x2
18x3y3,
1
2x2y=
9xy2
2x2y•9xy2=
9xy2
18x3y3,
1
9x3y=
2y2
9x3y•2y2=
2y2
18x3y3;
(2)∵最简公分母是(a-b)(a+b)2,
∴
1
(a+b)2=
a−b
(a+b)2(a−b),
2
−a+b=-
2
a−b=
2(a+b)2
(a+b)2(a−b),
3
a2−b2=
3
(a+b)(a−b)=
3(a+b)
(a+b)2(a−b).