>15

[C.851]
　b^2 +c^2 +bc = (3/4)(b+c)^2 + (1/4)(b-c)^2 ≧ (3/4)(b+c)^2,
　a√(b^2 +c^2 +bc) ≧ {(√3)/2}(ab+ca),
循環的にたす。


[C.844]
Σ[k=1,n] 1/k -γ -log(n) = ε(n) とおくと,
Σ[n=1,N] ε(n)/n = log(N)ε(N) + (1/2)ε(N)^2 - (1/2)γ^2 + (1/2)Σ[n=1,N] (1/n^2) + {(1/2)log(N)^2 - Σ[k=1,N] log(k)/k }
　→　-(1/2)γ^2 + (1/2)ζ(2) - L　(N→∞).