>>132
(dt/dx) = {x/√(x^2 +1)} +1 = t/√(x^2 +1)
t/dt = dx/√(x^2 +1)

∫1/√(x^2+1)dx　= ∫(1/t) dt = log|t| +c = log|√(x^2 +1) +x| +c