解1题：最简公分母是(x+y)
x-2y
=(x-2y)(x+y)/(x+y)
y²/(x+y)与(x-2y)(x+y)/(x+y)有相同的分母
解2题：最简公分母是(a-b)(b-c)(a-c)
1/[(a-b)(b-c)]
=(a-c)/[(a-b)(b-c)(a-c)]
1/[(b-c)(c-a)]
=-1/[(b-c)(a-c)]
= -(a-b)/[(a-b)(b-c)(a-c)]
=(b-a)/[(a-b)(b-c)(a-c)]
1/[(a-c)(a-b)]
=(b-c)/[(a-b)(b-c)(a-c)]