Real numbers include all rational and irrational numbers. There are several key properties and theorems regarding real numbers: 1) Euclid's division lemma states that for any two positive integers a and b, there exist unique integers q and r such that a = bq + r, where 0 ≤ r < b. This lemma is used to calculate the highest common factor of two integers. 2) The fundamental theorem of arithmetic states that every integer greater than 1 can be written as a unique product of prime factors, regardless of their order. 3) Rational numbers are numbers that can be written as a ratio of two integers. Irrational numbers are all other real numbers that are not rational and have non-repe