²=ac
≤ (a²+c²)/2
即 2b²≤ a²+c²
cosB=(a²+c²-b²)²/2ac
a²+c²-b²≥b²
cosB≥1/2
B∈ (0,60]
y=(sinB*cosB)/(1+sinB+cosB)
= [(sinB+cosB)²/2 - 1/2]/(1+sinB+cosB)
设sinB+cosB=t = √2sin(B+45)
故t ∈ (1,√2]
y= [t²/2 - 1/2]/ (1+t)
y= 1/2(t²-1)/(1+t)
=1/2* (t-1)
y函数单调递增
故y ∈ (0,1/2* (√2-1)]