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!

Any technique or trick for finding the coefficient

  1. Nov 8, 2009 #1
    any technique or "trick" for finding the coefficient

    Here is the polynomial expression


    Each x term is raised to ascending powers of 1.

    The entire sum in the brackets is raised to the 5th power.

    Does anyone have any "special trick" for finding the coefficient of the term which contains x^19?
  2. jcsd
  3. Nov 8, 2009 #2
    Re: any technique or "trick" for finding the coefficient

    what about generating functions? there's another way to rewrite the finite geometric series inside the brackets, and that formula raised to the 5th power has another formula, and you can plug in the appropriate numbers to get the coefficient for x^19
  4. Nov 9, 2009 #3

    Gib Z

    User Avatar
    Homework Helper

    Re: any technique or "trick" for finding the coefficient

    Think about how would you write out the expansion normally? You would write out the 5 factors in 5 seperate brackets (..)(..)(..)(..)(..), and pick a term from each bracket to form a term on the RHS. How many ways can you pick those terms differently to get the exponents adding to 19? It's now a combinatorics problem.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook