y=x^2/(1+x^4）=1/(x^2+1/x^2)
∵(x-1/x)^2≥0,∴x^2+1/x^2≥2
∴y=x^2/(1+x^4）=1/(x^2+1/x^2)≤1/2
又：y=x^2/(1+x^4)≥0
∴y值域：【0,1/2】
ab=a+b+3
ab-3=a+b
(ab)^2-6ab+9=a^2+b^2+2ab
(ab)^2-8ab+9=a^2+b^2
又：a^2+b^2≥2ab
∴(ab)^2-8ab+9≥2ab
(ab)^2-10ab+9≥0
(ab-1)(ab-9)≥0
ab≤1或ab≥9