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The sum of series (1)/(1.2) -(1)/(2....

The sum of series
` (1)/(1.2) -(1)/(2.3) + (1)/(3.4) - (1)/(4.5) + …….` is

A

`(1)/(n+1)`

B

`1-(1)/(n+1)`

C

`(1)/(n+1)-1`

D

`1+(1)/(n+1)`

Text Solution

Verified by Experts

The correct Answer is:
B
Let `T_(r)" be the "r^(th)` term of the given series. Then,
`T_(r)=(1)/(r(r+1)),r=1,2, . . .,n`
`rArr" "T_(r)=(1)/(r)-(1)/(r+1),r=1,2, . . .,n`
Let S be the required sum. Then,
`S=underset(r=1)overset(n)sumT_(r)=underset(r=1)overset(n)sum((1)/(r)-(1)/(r+1))`
`rArr" "S=(1-(1)/(2))+((1)/(2)-(1)/(3))+((1)/(3)-(1)/(4))+ . . . . +((1)/(n)-(1)/(n+1))`
`rArr" "S=1-(1)/(n+1)`
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OBJECTIVE RD SHARMA ENGLISH-SEQUENCES AND SERIES-Chapter Test
  1. The sum of series (1)/(1.2) -(1)/(2.3) + (1)/(3.4) - (1)/(4.5) + …...

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  2. Let H(n)=1+(1)/(2)+(1)/(3)+ . . . . .+(1)/(n), then the sum to n terms...

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  3. Sum of the first n terms of the series 1/2+3/4+7/8+(15)/(16)+............

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  4. If A(1),A(2) are between two numbers, then (A(1)+A(2))/(H(1)+H(2)) is ...

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  5. If the (m+1)t h ,(n+1)t h ,a n d(r+1)t h terms of an A.P., are in G.P....

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  6. Given that n arithmetic means are inserted between two sets of numbers...

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  7. If a,b, and c are in G.P then a+b,2b and b+ c are in

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  8. If in a progression a1, a2, a3, e t cdot,(ar-a(r+1)) bears a constant...

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  9. If in an AP, t1 = log10 a, t(n+1) = log10 b and t(2n+1) = log10 c then...

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  10. Find the sum of the series: 1^2-2^2+3^2-4^2+.....-2008^2+2009^2.

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  11. If 4a^(2)+9b^(2)+16c^(2)=2(3ab+6bc+4ca)," where "a,b,c are non-zero nu...

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  12. If Sn denotes the sum of n terms of an A.P. whose common difference is...

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  13. The sides of a right angled triangle are in A.P., then they are in the...

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  14. Find the sum of all the 11 terms of an AP whose middle most term is 30...

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  15. The maximum sum of the series 20+19 1/3+18 2/3+ is 310 b. 300 c. 0320 ...

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  16. If three numbers are in G.P., then the numbers obtained by adding the ...

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  17. If p ,q ,r are in A.P., show that the pth, qth and rth terms of any G....

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  18. Let a,b,c be three positive prime number. The progrrssion in which sqr...

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  19. If 1/(b-a)+1/(b-c)=1/a+1/c , then (A). a ,b ,a n dc are in H.P. (B). a...

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  20. If three numbers are in H.P., then the numbers obtained by subtracting...

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  21. The first three of four given numbers are in G.P. and their last three...

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