Links
[1] https://www.hootsgo.org/?q=node/6726
[2] http://www.cut-the-knot.org/m/Probability/SymmetryPrinciple.shtml
[3] http://www.cut-the-knot.org/Probability/dAlembert.shtml
[4] http://www.cut-the-knot.org/m/Probability/RandomPointsOnSegment.shtml