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I am pretty much clueless with this but would like some help the question i have been given is........How many terms are in the expansion of (a+b+c)^5 after like terms have been combined? Please help me all the way through it if you can. Thankyou

  • Math -

    The number of terms in a multinomial expansion of m terms raised to the nth power is equal to C(n+m-1,n).

    In the present case,
    n=5, m=3, so
    C(n+m-1,n)=C(5+3-1,5)=C(7,5)=7*6/(2*1)=21

    In the same vein, you may be interested to know that the individual coefficients for the terms
    a^p*b^q*c^r (where p+q+r=n)
    is
    C(n,p,q,r)
    =n!/(p!q!r!)
    For example, the term a²b²c
    has p=2,q=2,r=1
    so the coefficient is
    C(5,2,2,1)
    =5!/(2!2!1!)

    For more interesting facts, see:
    http://en.wikipedia.org/wiki/Multinomial_theorem

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