n = 12
Sum of A.P., 2 + 4 + 6 + ... + 24 =^122 ()^2 +^24
= 6 (26)
2 + 4 + 6 + ... + 24 = 156
The idea needs to be explained through linear presentation of the text material,
laying emphasis on the knowledge of the different symbols used in the formula.
- Geometric Progression (GP)
Sequences in which the ratio of each term to its predecessor is always same are
called as geometric sequences or geometric progressions, denoted as GP.
Eg. : 1, 2, 4, 8, ...
5, 15, 45, 135, ...
The idea needs verbal explanation supported by relevant text material.
- Common ratio
In a GP the ratio of each term to its predecessor which is always same is called as the
common ratio and is normally denoted by the letter ‘r’.
- General term of a GP
The terms of a GP are normally denoted as t 1 , t 2 , t 3 , t 4 , ... and the nth term is denoted
as tn. The nth term of a G.P is called as the general term of a GP and can be found using
the formula,
tn = arn-1
The idea may be explained through written material in Braille in linear format.
Eg. : Assume that the sequence is 1, 3, 9, 27, .... and has 6 terms. The general term
which is the last of the series can be found out using the formulae.
tn =1 x 36-1
=1 x 3^5
