令t=√x，x=t²，t∈[0,1]dx=2t∫[0->1] √x/(1+x)dx=2∫[0->1] t²/(1+t²)dt=2∫[0->1] 1-[1/(1+t²)]dt=2-2arctant | [0->1]=2-π/2
1-ln2
