※ 引述《hex8828 (D202陈先生)》之铭言：
: https://i.imgur.com/HQ7XiZC.jpg
xdx = (1/2)d(x^2)
u = x^2
1/(1 + u^2) du = darctan(u)
: https://i.imgur.com/BhN4v4R.jpg
tan 60度 = sqrt(3)
tan 30度 = 1/sqrt(3)