1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Urgent help required in summation of terms

  1. May 20, 2013 #1
    Find the sum upto n terms:


    I solve it this way:

    Now can I take summation on both sides? How?

    I mean when I add 2 on both sides the resultant is 0(2-2=0).Similarly the resultant summation will be zero?

    And if I take summation I get one term as 3Ʃ.Now in a book I saw that it is 3n. Why? Summation of 3 will be 3 only as 3 is constant.Please explain.

    I got this:


    Ʃtn=Ʃ[8n^3 + 12n^2 - 2n - 3]

    Ʃtn=Ʃ[8n^3] + Ʃ[12n^2] - Ʃ[2n] -Ʃ[3]

    Ʃtn=8*Ʃ[n^3] + 12*Ʃ[n^2] - 2*Ʃ[n] - Ʃ[3]

    Now do I require to write them as Ʃ[3] or 3Ʃ (putting a constant outside Ʃ).Please explain the whole summation process.I am stuck here.
  2. jcsd
  3. May 20, 2013 #2

    You are on the right way.

    As for Ʃtn=8*Ʃ[n^3] + 12*Ʃ[n^2] - 2*Ʃ[n] - Ʃ[3], there are formula for Ʃ[n^3], Ʃ[n^2], Ʃ[n] and Ʃ[1].
  4. May 20, 2013 #3

    Stephen Tashi

    User Avatar
    Science Advisor

    No. Summing a constant depends on how many times you sum it.

    For example,

    [itex] \sum_{i=1}^4 10 = 10 + 10 + 10 + 10 = (4)(10) = 40 [/itex]
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook