Question

一道高数题，划线的步骤不懂，求解答！！

Answer 1

let
x=tanu
dx= (secu)^2 du
x=0, u=0
x=1, u=π/4
∫(0->1) dx/√(1+x^2)
=∫(0->π/4) secu du
=[ln|secu+tanu|]|(0->π/4)
=ln(√2 +1)