(1)x/6ab^2,y/9a^2bc 的公分母是18a^2b^2 c
所以x/6ab^2=3acx/18a^2b^2 c
y/9a^2bc =2yb/18a^2b^2 c
通分就是找它们（分母）的公因式
(2）∵a^2+2a+1=(a+1)^2
a^2-1=(a+1)(a-1)
∴公分母是(a+1)^2(a-1)
a-1/a^2+2a+1
=(a-1)(a-1)/(a+1)^2(a-1)
=(a^2-1)/(a^3+a^2-a-1)
6/a^2-1
=6(a+1)/(a^2-1)(a+1)
=(6a+6)/(a^3+a^2-a-1)