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The sum of the first three terms of an i...

The sum of the first three terms of an increasing G.P. is 21 and the sum of their squares is 189. Then the sum of its first n terms is

A

`3(2^(n) -1)`

B

`12(1 - (1)/(2^(n)))`

C

`6(1 - (1)/(2^(n)))`

D

`6 (2^(n) -1)`

Text Solution

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The correct Answer is:
A
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ALLEN-SEQUENCE AND PROGRESSION-Exercise O-2
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  2. If the sum of the first 11 terms of an arithmetical progression equals...

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  3. Let s(1), s(2), s(3).... and t(1), t(2), t(3).... are two arithmetic s...

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  4. If x in R , the numbers 5^(1+x) + 5^(1-x) , a/2 , 25^x + 25^(-x) form ...

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  5. Along a road lies an odd number of stones placed at intervals of 10m. ...

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  6. In an A.P. with first term 'a' and the common difference d(a, d!= 0), ...

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  7. Let an, n in N is an A.P with common difference d and all whose terms ...

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  8. Let a(1), a(2), a(3).... and b(1), b(2), b(3)... be arithmetic progres...

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  9. The arithmetic mean of the nine number in the given set {9,99,999, ......

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  10. If (1 + 3 + 5 + .... " upto n terms ")/(4 + 7 + 10 + ... " upto n term...

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  11. If a != 1 and l n a^(2) + (l n a^(2))^(2) + (l n a^(2))^(3) + ... = 3 ...

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  12. The sum of the first three terms of an increasing G.P. is 21 and the s...

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  13. a, b, c are distinct positive real in HP, then the value of the expres...

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  14. If sin(x-y),sinx,sin(x+y) are in H.P., then find the value of sinxsec(...

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  15. An H.M. is inserted between the number 1/3 and an unknown number. If w...

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  16. lf abcd=1 where a,b,c,d are positive reals then the minimum value of ...

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  17. If 27 abc> (a+b+c)^3 and 3a +4b +5c=12 then 1/a^2+1/b^3+1/c^5=10, wh...

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  18. If x = sum(n=0)^(oo) a^(n), y=sum(n=0)^(oo) b^(n), z = sum(n=0)^(oo) ...

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  19. IF S=1^2+3^2+5^2....99^2 then the value of the sum 2^2+4^2+6^2...100^2

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  20. For which positive integers n is the ratio, (sum+(k=1)^(n) k^(2))/(sum...

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