分式(x^2-1)/[(x-1)(x+2)] =(x+1)(x-1) /[(x-1)(x+2)]
有意義;
[(x-1)(x+2)]≠0
(x-1)≠0 x ≠1
(x+2)≠0 x ≠-2
無意義;
(x-1)(x+2)]=0
(x-1)=0 x =1
(x+2)=0 x =-2
等于0
(x+1)(x-1)=0
x=-1