∵a2+b2+c2=a2+ 1 10 b2+ 9 10 b2+c2=(a2+ 1 10 b2)+( 9 10 b2+c2)≥2 a2? 1 10 b2 +2 9 10 b2c2 =2 1 10 ab+6 1 10 bc= 10 5 (ab+3bc),∴ a2+b2+c2 ab+3bc ≥