>>739
(3)
a^4 - (a^3+b^3)(a-kb) = {k(a^3+b^3) -abb} b
　= {k[a^3 +(1/2)b^3 +(1/2)b^3] -abb} b
　≧ {3k/(2^(2/3)) -1} ab^3,　　　　(AM-GM)
(係数) ≧0 より
　k = (1/3)･2^(2/3) = 0.529133684

　a^4/(a^3 + b^3) ≧ a - kb,
循環的にたす。
　(左辺) ≧ (1-k)(a+b+c) = 0.470866316 (a+b+c).