Login

If the sum of first 7 terms of an AP is 49 and that of first 17 terms is 289.Find the sum of first n terms

S 7 = 7/2 [2a+(6)d]

49* 2/7 = 2a+6d

7*2 = 2a+6d

14 = 2a+6d........(1)

s17= 17/2 (2a+16d)

289*2/17 = 2a+16d

17*2 = 2a + 16d

34 = 2a + 16d .........(2)

subtracting eq. 2 from 1 we will get

20 = 10 d

so ,d = 2

putting d = 2 in eq. 1

14 = 2a + 6 * 2

14 = 2a + 12

14 - 12 = 2a

2a = 2

a = 1

sn= n/2 [2a+ (n-1)d]

=n/2[2(n)]

=n2

sn =n2

so sum of first natural number = n2

  • 138
View Full Answer

YOU ARE RIGHT.................

  • -20

I HAVE THUMBS UP YOUR ANSWER NOW THUMBS UP MINE ALSO..........

  • -11
wow great u have helped me a lot mohil
  • 8
Corrrct! Helped me for my boards☺
  • -9
I done this in other method but it is also correct can you plz check it.. 7a+21d=49 17a+136d=289 Solving these we get, a=1 and d=2
  • -11
n2 is the sum of n terms of this particular a.p
  • -13
Here it is...

  • 65
!!
  • -10
If x square is equals to 2 by 3 and X = 2 - 3 are roots of the quadratic equation X square + 7 x + 20 20 find the values of a and b
  • -8
Please answer for this question
  • -12
Thanks, it helps alot
  • -11
I think it is correct

  • -12
Check if it's right

  • 25
answer is here...

  • 3
Hope dis helped you a lot.....

  • 4
I hope u understand this question 👍👍

  • 5
All Answers correct
Great job!!
  • -2
S 7 = 7/2 [2a+(6)d] 49* 2/7 = 2a+6d 7*2 = 2a+6d 14 = 2a+6d........(1) s17= 17/2 (2a+16d) 289*2/17 = 2a+16d 17*2 = 2a + 16d 34 = 2a + 16d .........(2) subtracting eq. 2 from 1 we will get 20 = 10 d so ,d = 2 putting d = 2 in eq. 1 14 = 2a + 6 * 2 14 = 2a + 12 14 - 12 = 2a 2a = 2 a = 1 sn= n/2 [2a+ (n-1)d] =n/2[2(n)] =n2 sn =n2 so sum of first natural number = n2
  • 2
S 7 = 7/2 [2a+(6)d] 49* 2/7 = 2a+6d 7*2 = 2a+6d 14 = 2a+6d........(1) s17= 17/2 (2a+16d) 289*2/17 = 2a+16d 17*2 = 2a + 16d 34 = 2a + 16d .........(2) subtracting eq. 2 from 1 we will get 20 = 10 d so ,d = 2 putting d = 2 in eq. 1 14 = 2a + 6 * 2 14 = 2a + 12 14 - 12 = 2a 2a = 2 a = 1 sn= n/2 [2a+ (n-1)d] =n/2[2(n)] =n2 sn =n2 so sum of first natural number = n2
  • 0
equate the sn of 7 and 17 
ans= n^2
  • 0
What are you looking for?