证明：a,b,c为正实数，令a^2=b^2=c^2，则：a^2=b^2=c^2=1/3a(1-a^2)=√（1/3）*(1-1/3)=（√3）/3 *2/3=（2√3）/9而当a^2=b^2=c^2时a(1-a^2)最大，故：a(1-a^2)<=（2√3）/9
