∫√(x^2+1) dx
令x=tanz,dx=sec^2z dz
原式=∫sec^3z dz
=(1/2)tanzsecz+(1/2)∫secz dz
=(1/2)tanzsecz+(1/2)ln(secz+tanz)+C
=(1/2)x√(x^2+1)+(1/2)ln[x+√(x^2+1)]+C