### I. Factors and Multiples :

If a number a divides another number b exactly, we say that a
is a factor of b. In this case, b is called a multiple of a.

### II. Highest Common Factor (H.C.F.) or Greatest Common Measure (G.C.M.) or
Greatest Common Divisor (G.C.D.):

The H.C.F. of two or more than two numbers is
the greatest number that divides each of them exactly.

There are two methods of finding the H.C.F. of a given set of numbers :

### 1. Factorization Method :

Express each one of the given numbers as the product
of prime factors.The product of least powers of common prime factors gives H.C.F.

### 2. Division Method:

Suppose we have to find the H.C.F. of two given numbers.
Divide the larger number by the smaller one. Now, divide the divisor by the
remainder. Repeat the process of dividing the preceding number by the remainder
last obtained till zero is obtained as remainder. The last divisor is the required
H.C.F.

### Finding the H.C.F. of more than two numbers :

Suppose we have to find the
H.C.F. of three numbers. Then, H.C.F. of [(H.C.F. of any two) and (the third
number)] gives the H.C.F. of three given numbers.
Similarly, the H.C.F. of more than three numbers may be obtained.

###
III. Least Common Multiple (L.C.M.) :

The least number which is exactly divisible by
each one of the given numbers is called their L.C.M.

#### 1. Factorization Method of Finding L.C.M.:

Resolve each one of the given
numbers into a product of prime factors. Then, L.C.M. is the product of highest
powers of all the factors.

#### 2. Common Division Method {Short-cut Method) of Finding L.C.M.:

Arrange the given numbers in a row in any order. Divide by a number which
divides exactly at least two of the given numbers and carry forward the numbers
which are not divisible. Repeat the above process till no two of the numbers are
divisible by the same number except 1. The product of the divisors and the
undivided numbers is the required L.C.M. of the given numbers.

### IV. Product of two numbers =Product of their H.C.F. and L.C.M.

### V. Co-primes:

Two numbers are said to be co-primes if their H.C.F. is 1.