y = (2-x²)/(3x²+5)
= 2/(3x²+5) - (x²+5/3-5/3)/(3x²+5)
= 2/(3x²+5) - 1/3 + (5/3)/(3x²+5)
= (11/3)/(3x²+5) - 1/3
3x²+5≥5
0≤1/(3x²+5)≤1/5
0≤(11/3)/(3x²+5)≤11/15
-1/3 ≤ (11/3)/(3x²+5) - 1/3 ≤ 2/5
值域：【-1/3,2/5】