Sum of a number of terms in g.p., Mathematics

We know that the terms in G.P. are:

a, ar, ar2, ar3, ar4, ................, arn-1

Let s be the sum of these terms, then

s = a + ar + ar2 + ar3 + ar4 + ................+ arn-1

                            or

s

 = 1058_GP.png

This is obtained as follows:

We know that

s = a + ar + ar2 + ar3 + ar4 +.............+ arn - 1                                            ......(1)

Multiplying this with "r" throughout, we have

r.s     = r.a + r.ar + r.ar2 + r.ar3 + r.ar4 +........+ r.arn-1 

         = ar + ar2 + ar3 + ar4 + ar5 +.......+ arn                                             ......(2)

Subtracting (1) from (2), we have

r.s - s = (ar - a) + (ar2 - ar) + (ar3 - ar2) +.......+ (arn - arn-1)

After canceling the terms equal in magnitude but opposite in sign, we are left with

s(r - 1) = arn - a
s(r - 1) = a(rn - 1)
 or s    = 427_GP1.png

By changing the signs in the numerator and the denominator we can also write the above equation as

 s  = 1058_GP.png

What happens to the above formula if the value of n is very large. The above formula can be written as

s

916_GP2.png

As the value of n approaches infinity (very large) the expression  2050_GP3.png becomes smaller to that extent where we ignore it. In this case the nth term is given as

Tn

= 1193_GP4.png

Now we look at a couple of examples.

Example

Find the sum of the series which is given below to 13 terms.

                   81, 54, 36, .............

The first term 'a' = 81 and the common ratio is obtained from the ratio of 54 and 81 or 36 and 54. It is 54/81 = 2/3. Now we employ the formula given above to calculate the sum of series to 13 terms.

s =   

1772_GP5.png

       = 241.78

The same series if considered as an infinite series, the sum of n terms would be

T = 491_GP6.png

= 243
Posted Date: 9/13/2012 4:36:13 AM | Location : United States







Related Discussions:- Sum of a number of terms in g.p., Assignment Help, Ask Question on Sum of a number of terms in g.p., Get Answer, Expert's Help, Sum of a number of terms in g.p. Discussions

Write discussion on Sum of a number of terms in g.p.
Your posts are moderated
Related Questions
please help me in my assignment, explain Applications of Markov Chains in Business.

3/5 of the soda purchased at the football game was cola. What percentage of the soda purchased was cola? Change the fraction to a decimal through dividing the numerator through

Determine the Projection of b = (2, 1, -1) onto a = (1, 0, -2) There is a requirement of a dot product and the magnitude of a. a →  • b → = 4                             ||a

The region bounded by y=e -x and the x-axis among x = 0 and x = 1 is revolved around the x-axis. Determine the volume and surface area of this solid of revolution.


Determine if the following sequences are monotonic and/or bounded. (a)   {-n 2 } ∞ n=0 (b) {( -1) n+1 } ∞ n=1 (c) {2/n 2 } ∞ n=5 Solution {-n 2 } ∞ n=0

Bayes’ Theorem In its general form, Bayes' theorem deals with specific events, such as A 1 , A 2 ,...., A k , that have prior probabilities. These events are mutually exclusive



E1) Create a guessing game for children of Class 2, to familiarise them with the concept of a time interval E2) How could you use group dancing to teach concepts of geometry? Th