y=（x²-3x+2）/ （x²+2x-3）
= (x-1)(x-2)/[(x+3)(x-1)]
= (x-2)/(x+3)
= (x+3-5)/(x+3)
= 1 - 5/(x+3)
-5/(x+3)≠0,1 - 5/(x+3)≠1
值域（-∞,1）,（1,+∞）