dx /√(x^2+1)
=∫ [x+√(x^2+1)] /{√(x^2+1)*[x+√(x^2+1)]} dx

=∫ [1 + x/√(x^2+1)]dx /[x+√(x^2+1)]
=∫ d[x + √(x^2+1)] /[x+√(x^2+1)]
= ln[x+√(x^2+1)] + C