y=2/(x²-3x+2)
定义域只要
x²-3x+2≠0
(x-1)(x-2)≠0
解得(x≠1),(x≠2)
如果是
y=√(x²-2x-15)
x²-2x-15≥0
(x-5)(x+3)≥0
x≥5
或x≤－3