Cutting a Stick to form a Triangle

Q: A stick is cut at two points uniformly randomly. What is the probability that the three sticks will make a triangle.

A: This problem is best solved by taking a geometrical approach. Without loss of generality, we can assume that the stick has unit length.  Let the cut be made at points A & B. The points A & B can take values uniformly between 0 and 1. The rule for any three segments making a triangle is that the sum of any two sides should be greater than the third. The total number of possible ways can be thought to be the set of points within a unit square as shown below.

For any given point (A,B), the stick segments are a, (b-a)  and (1-b), we have three inequalities that need to be simultaneously satisfied (if we assume a < b).

The simplification of the above set of equation yields

The eq 1 & 2 correspond to the top left quadrant of the unit square, while eq 3 corresponds to the area under the diagonal in that quadrant as shown in the figure below.

The greyed out region is the feasible region. This region represents 1/8th the total area. Remember we assumed a < b, and things would be just the same if a > b. Thus the total feasible area is 1/4 which is the sought probability

Some books on probability/algorithms
Fifty Challenging Problems in Probability with Solutions (Dover Books on Mathematics)
This book is a great compilation that covers quite a bit of puzzles. What I like about these puzzles are that they are all tractable and don't require too much advanced mathematics to solve.

Introduction to Algorithms
This is a book on algorithms, some of them are probabilistic. But the book is a must have for students, job candidates even full time engineers & data scientists

Introduction to Probability Theory

An Introduction to Probability Theory and Its Applications, Vol. 1, 3rd Edition

The Probability Tutoring Book: An Intuitive Course for Engineers and Scientists (and Everyone Else!)

Introduction to Probability, 2nd Edition

The Mathematics of Poker
Good read. Overall Poker/Blackjack type card games are a good way to get introduced to probability theory

Let There Be Range!: Crushing SSNL/MSNL No-Limit Hold'em Games
Easily the most expensive book out there. So if the item above piques your interest and you want to go pro, go for it.

Quantum Poker
Well written and easy to read mathematics. For the Poker beginner.


Bundle of Algorithms in Java, Third Edition, Parts 1-5: Fundamentals, Data Structures, Sorting, Searching, and Graph Algorithms (3rd Edition) (Pts. 1-5)
An excellent resource (students/engineers/entrepreneurs) if you are looking for some code that you can take and implement directly on the job.

Understanding Probability: Chance Rules in Everyday Life A bit pricy when compared to the first one, but I like the look and feel of the text used. It is simple to read and understand which is vital especially if you are trying to get into the subject

Data Mining: Practical Machine Learning Tools and Techniques, Third Edition (The Morgan Kaufmann Series in Data Management Systems) This one is a must have if you want to learn machine learning. The book is beautifully written and ideal for the engineer/student who doesn't want to get too much into the details of a machine learned approach but wants a working knowledge of it. There are some great examples and test data in the text book too.

Discovering Statistics Using R
This is a good book if you are new to statistics & probability while simultaneously getting started with a programming language. The book supports R and is written in a casual humorous way making it an easy read. Great for beginners. Some of the data on the companion website could be missing.