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# The common ratio , the last term , and the sum of G.P. is 3, 486 and 728. Find first and the number of terms. Grade:Upto college level

## 3 Answers

10 years ago

r = 3 , last term = 486  ,  sum = 728

last term = arn-1 = 486    ..................1

sum = a(rn - 1)/r-1 = 728     ................2

divide both

728/486 = (rn-1)/[(r-1)(rn-1)

r = 3 so

728/486 = 3n-1/2x3n-1

729 - 1 / 243 = 3n - 1 /3n-1

36 - 1 / 35  =  3n - 1 / 3n-1

comparing both sides we get

n = 6 or n-1 = 5

total terms = 6

from eq 1 , arn-1 = 486

a 35 = 486 = 243x2 = 35x2

a = 2

firat term is 2 & total terms are 6 ,

approve if u like my ans

10 years ago Please approve

10 years ago

Hello Sir,

Please give me the answer for this question from Arithmetic Progression.

Insert a A.M. between 5 and 20.

Anitha.

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