SOLUTION: I would be thankful if someone kindly help me with this problem:
In an AP 3,7,11,... find the value of n for which the sum of the first 2n terms will exceed the sum of first n ter
Question 917875: I would be thankful if someone kindly help me with this problem:
In an AP 3,7,11,... find the value of n for which the sum of the first 2n terms will exceed the sum of first n terms by 155.
I tried solving it as follows:
S2n = Sn + 155
2n/2 {2(3) + (n-1)(4)} = n/2 {2(3) + (n-1)(4)} + 155
Proceeding further I finally got:
2n^2 + n -155 = 0
This equation does not give the value of n as whole number. Answer by Edwin McCravy(20056) (Show Source): You can put this solution on YOUR website!
Your error is here:
2n/2 {2(3) + (n-1)(4)}
Should be
2n/2 {2(3) + (2n-1)(4)}
because you are finding the "2n"th term.
That way you get
6n^2 + n -155 = 0 instead of 2n^2 + n -155 = 0
which factors.
Edwin