2*(1-a^2)=b^2
√a*(1+2-2a^2)=y
y'=(3/2)/√a-(5/2)a^(3/2)=0
3/2=(5/2)*a^2
a^2=3/5
a=√(3/5)时,最大值ymax=√0.6*(3-2*3/5)
=9√15/5