# Math

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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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