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Sequence and Series Class 11 | Ch-8

Sequence And Series 

Geometric Progression

Geometric Progression, Its nth term, Sum to n terms, sum to infinite terms, Properties of G.P.,Special Results

GEOMETRIC PROGRESSION
A sequence of non-zero numbers is called a geometric progression (G.P.) if the ratio of the term and the term preceding to it is always a constant quantity.

A sequence a1, a2, a3, a4, ………an, an+1  is called geometric progression. 
If   is the common ratio of GP. Where  
General Geometric Progression is  a, ar, ar2, ar3, ……….., arn-1

First term = a, Second term = ar,  and so on and  r is the common ratio of GP
nth term in GP is =  arn-1 
Sum of first n terms in GP is 
Sum of first n terms in GP is
Sum to infinite terms in GP.
General GP with infinite number of terms is of the following type
ar, ar2, ar3, ……….., arn-1 .........∞
Sum to infinite terms of GP is given by
SELECTION OF TERMS IN G.P.
\[Three\; terms\; in\; \; G.P.\; are:\; \; \; \frac{a}{r},\; a,\; ar, \; \; here \; common \; ratio\; is\; r\]
\[Four\; terms\; in\; \; G.P.\; are:\; \; \; \frac{a}{r^{3}},\; \frac{a}{r},\; ar, \; ar^{3},\;\; here \; common \; ratio\; is\; r^{2}\]\[Five\; terms\; in\; \; G.P.\; are:\; \; \; \frac{a}{r^{2}},\; \frac{a}{r}, a,\; ar, \; ar^{2},\;\; here \; common \; ratio\; is\; r\]
PROPERTIES OF GEOMETRIC PROGRESSION:- 
1) If all the terms of G.P.  be  multiplied  or  divided  by  the  sane  non- zero  constant,    then  it remains a   G.P.   with  common  ratio  r.
2) Reciprocal of the terms of a G.P. form a G.P.
3) If each term of a G.P. raised to the same power , the resulting sequence also form a G.P.
\[4)\; If a_{1},\; a_{2},\; a_{3},........\; a_{n}.... are\; the \; terms \; of \; G.P.,\; \; then\;\\ log\; a_{1},log\; a_{2},.....log\; a_{n}........... \; \;\; \]\[are\; in\; A.P. \; \; and \; \; vice\; -\; versa\]
GEOMETRIC MEAN 
If a, b, c are the three terms of GP then b is said to be the Geometric Mean and is given by\[b=\sqrt{ac}\: \: or\: \: b^{2}=ac\]
Explanation:- 
If a, b, c are in GP then\[\frac{b}{a} = r = \frac{c}{b}\] ⇒ \[\frac{b}{a} = \frac{c}{b}\]\[b^{2}=ac\Rightarrow b=\sqrt{ac}\]

Componendo and Dividendo
If four terms a, b, c, d are proportional then \[\frac{a}{b}=\frac{c}{d}\]
When we apply componendo and dividendo then we get  \[\frac{a+b}{a-b}=\frac{c+d}{c-d}\]
\[or\; \; \frac{Numerator+Denomenator}{Numerator-Denomenator} =\frac{Numerator+ Denomenator}{Numerator-Denomenator}\]
SPECIAL RESULTS:-
Sum of first n  natural number  is given by
Sum of square of first n natural number is given by
Sum of cube of first n natural number is

Question:
The first term of an infinite G.P. is 1 and any term is equal to the sum of all terms that follow it. Find the infinite G.P.
Solution:  It is given that:   a = 1

A.T.Q.        Tn = Tn+1 + Tn+2 + Tn+3 +  ……………………

                  arn-1 = arn + arn+1 + arn+2 + ……………………

\[ar^{n-1}=\frac{ar^{n}}{1-r}\]Now cross multiply it  and putting a = 1 we get \[r^{n-1}-r^{n}=r^{n}\Rightarrow 2r^{n}=r^{n-1}\]\[\Rightarrow 2r = 1 \Rightarrow r=\frac{1}{2}\]

Putting a = 1 and r = 1/2 we get the required sequence as follows \[\frac{1}{2},\; \frac{1}{4},\; \frac{1}{16},\; .......,\; \infty\]


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