m^2+mn≤a(m^2+n^2)
(a-1)m^2-mn+an^2>=0
∵恒成立
∴a-1>0 开口向上
(-1)²-4(a-1)a1
由(2)得
4a²-4a-1>=0
a>=1/2+√2/2 或a=1/2+√2/2
∴最小的a值是1/2+√2/2