# Summary

- A sequence is an arrangement of numbers in a definite order according to some rule.
- If is a sequence, is the corresponding series.
- If the number of terms in a sequence or series is finite, we call it a finite sequence (series), otherwise it is infinite sequence (series).
- If an Arithmetic Progression (A.P), every term either increases or decreases by a fixed quantity called the common difference.

If is the term and , the common differences,

where is the number of terms;

A.M between - In a geometric progression (G.P) every term is either multiplied or divided by a fixed non-zero quantity to get the next term. This quantity is called the common ratio.
- If is the term and the common ratio, then

G.M =

where a and b are the numbers between which G.M found. - A harmonic progression (H.P) is formed by the reciprocals of an A.P.

forms a H.P

H.M between = . - If A, G, H are the A.M, G.M and H.M between any two numbers, then

- Miscellaneous series: