换元法
令√t=x
t=x^2
t=0,x=0
t=1,x=1
dt=2xdx
原式=∫[0,1] 2xdx/(1+x)
=2∫[0,1] x(1+x)dx
=2∫[0,1] (x+1-1)/(1+x)dx
=2∫[0,1] [1-1/(1+x)]dx
=2[x-ln(1+x)] [0,1]
=2-2ln2
换元法
令√t=x
t=x^2
t=0,x=0
t=1,x=1
dt=2xdx
原式=∫[0,1] 2xdx/(1+x)
=2∫[0,1] x(1+x)dx
=2∫[0,1] (x+1-1)/(1+x)dx
=2∫[0,1] [1-1/(1+x)]dx
=2[x-ln(1+x)] [0,1]
=2-2ln2