a=√(e^x-1)
x=ln(a²+1)
dx=2a/(a²+1) da
x=0,a=0
x=ln2,a=1
原式=∫(0,1) 2a²da/(a²+1)
=2∫(0,1) [1-1/(a²+1)]da
=2(a-arctana) (0,1)
=2(1-π/4)