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A wide variety of questions in mathematics starts with the question ’Is it possible...?’. In such problems you would either present an example, in case the described situation is possible, or rigorously prove that the situation is impossible, with the help of counterexample or by any other means. Sometimes the border between what seems should be possible and impossible is not immediately obvious, therefore you have to be cautious and verify that your example (or counterexample) satisfies the conditions stated in the problem. When you are asked the question whether something is possible or not and you suspect it is actually possible, it is always useful to ask more questions to gather additional information to narrow the possible answers. You can ask for example "How is it possible"? Or "\(\bf Which\) properties should the correct construction satisfy"?

Welcome back! We hope you all had a great summer and now you are ready for the new school year full of fun problems in mathematics. We decided to start with warm-up topic called dissections, so today we will cut various shapes into more elaborate geometric figures in order to reassemble them into a different shape.

Today we will practice to encrypt and decipher information using some of the most common codes. Majority of the codes in use can be alphabetic and numeric, namely one may want to encode a word, a phrase, or a number, or just any string of symbols using either letters, or numbers, or both. Some of the codes, however may use various other symbols to encrypt the information. To solve some of the problems you will need the correspondence between alphabet letters and numbers

Long ago in a galaxy far away there was a planet of liars and truth tellers, it is known that liars always tell lie, and truth tellers always respond with correct statements. All the inhabitants of the planet look identical to each other, so there is no way to distinguish between liars and truth tellers just by looking at them.

The planet is ruled by the government, where one may encounter honest governors as well as liars. The government is controlled by the High Council, where again one can meet liars and truth tellers.

Last weekend we held the verbal challenge and today we decided to demonstrate solutions of the most juicy problems.

Today we will focus on the study of Euclidean geometry of plane figures. Around 300 BCE a Greek mathematician Euclid developed a rigorous way to study plane geometry in his work *Elements* based on axioms (statement assumed to be correct) and theorems (statements deduced from axioms). The axioms of Euclidean *Elements* are the following:

For any two different points, there exists a line containing these two points, and this line is unique.

A straight line segment can be prolonged indefinitely.

A circle is defined by a point for its centre and a distance for its radius.

All right angles are equal.

For any line \(L\) and point \(P\) not on \(L\), there exists a line through \(P\) not meeting \(L\), and this line is unique.

In examples we deduce from the axioms above the following basic principles:

1. The supplementary angles (angles "hugging" a straight line) add up to \(180^{\circ}\).

2. The sum of all internal angles of a triangle is also \(180^{\circ}\).

3. A line cutting two parallel lines cuts them at the same angles (these are called corresponding angles).

4. In an isosceles triangle (which has two sides of equal lengths), the two angles touching the third side are equal.

Let’s have a look at some examples of how to apply these axioms to prove geometric statements.

Sometimes different areas in mathematics are more related than they seem to be. A lot of algebraic expressions have geometric interpretation, and a lot of them can be used to solve problems in number theory.

Today we will solve several logic problems that revolve about a very simple idea. Imagine you are in a room in a dungeon and you can see doors leading out of the room. Some of them lead to the treasure and some of them lead to traps. It is possible that all doors lead to treasure or all lead to traps, but it is also possible that one door leads to treasure and all other lead to traps. Unless specified, there is always something behind the door.

Each door has a sign with a statement on it, but those statements are not always true. You have a dungeon guide, who is always honest with you and will tell you something about the truthfulness of the statements on the doors, but it will be up to you to put it all together and pick the correct door... or walk away, if you believe there is no treasure.

There exist various ways to prove mathematical statements, one of the possible methods, which might come handy in certain situations is called **Proof by contradiction**. To prove a statement we first assume that the statement is false and then deduce something that contradicts either the condition, or the assumption itself, or just common sense. Thereby concluding that the first assumption must have been wrong, so the statement is actually true.

Sometimes one can guess certain multiples of a number just by looking at it, the idea of this sheet is to learn to recognise quickly using tricks when a natural number is divisible by another number.