∵x^2-6x+17=(x-3)^2+8≥8
∴log2(x^2-6x+17)≥log2(8)=3
∴值域为：y≥3log2(x^2-6x+17)≥log2(8)=3他是怎么等于三的呢当x=3时，x^2-6x+17=8，y=3哈