>>236

解１
exp(xx) > 1+xx より
(与式) < ∫[0,a] 1/(1+xx)^2 dx
　= (1/2)a/(1+aa) + (1/2)arctan(a)
　< π/4
　= 0.7853981634

解２
GM-AM より　exp(-xx)/(1+xx) < {1/(1+xx)^2 + exp(-2xx)}/2,
(与式) < (1/4)a/(1+aa) + (1/4)arctan(a) + (1/8)√(2π)erf((√2)a)
　< {π + √(2π)}/8
　= 0.706027616

なお、真値は (eπ/2)･erfc(1) = 0.67164671