Problems

Now you have two doors with the statements:

1. It makes no difference which door you pick.

2. There is treasure behind the first door.

Your guide says: The first sign is true if there is treasure behind the first door, otherwise it is false. The second sign is false if there is treasure behind the second door, otherwise it is true. What do you do?

Now there are three doors with statements on them:

1. There is nothing behind the third door.

2. There is a trap behind the first door.

3. There is nothing behind this door.

Your guide says: There is treasure behind one of the doors, trap behind another one and there is nothing behind the third door. The sign on the door leading to treasure is true, the sign on the door leading to a trap is false, and the third sign might be true or false.
Which door will you open, if you really really want the treasure?

This is a famous problem, called Monty Hall problem after a popular TV show in America.
In the problem, you are on a game show, being asked to choose between three doors. Behind each door, there is either a car or a goat. You choose a door. The host, Monty Hall, picks one of the other doors, which he knows has a goat behind it, and opens it, showing you the goat. (You know, by the rules of the game, that Monty will always reveal a goat.) Monty then asks whether you would like to switch your choice of door to the other remaining door. Assuming you prefer having a car more than having a goat, do you choose to switch or not to switch?

Find a representation as a product of \(a^n - b^n\) for general \(a,b,n\).

Let \(a,b,c,d\) be positive real numbers. Prove that \((a+b)\times(c+d) = ac+ad+bc+bd\). Find both algebraic solution and geometric interpretation.

Let \(a,b,c,d\) be positive real numbers such that \(a\geq b\) and \(c\geq d\). Prove that \((a-b)\times(c-d) = ac-ad-bc+bd\). Find both algebraic solution and geometric interpretation.

Using the area of a rectangle prove that \(a\times b=b\times a\).

Jason has \(20\) red balls and \(14\) bags to store them. Prove that there is a bag, which contains at least two balls.

One of the most useful tools for proving mathematical statements is the Pigeonhole principle. Here is one example: suppose that a flock of \(10\) pigeons flies into a set of \(9\) pigeonholes to roost. Because there are \(10\) pigeons but only \(9\) pigeonholes, at least one of these \(9\) pigeonholes must have at least two pigeons in it.

Show the following: Pigeonhole principle strong form: Let \(q_1, \,q_2,\, . . . ,\, q_n\) be positive integers. If \(q_1+ q_2+ . . . + q_n - n + 1\) objects are put into \(n\) boxes, then either the \(1\)st box contains at least \(q_1\) objects, or the \(2\)nd box contains at least \(q_2\) objects, . . ., or the \(n\)th box contains at least \(q_n\) objects.
How can you deduce the usual Pigeonhole principle from this statement?