posted by Anonymous .
when expanding (x+y)^8, either an x or a y from each of the eight factors is used in the product. That means that x^3y^5 is going to occur several times and the number of times it occurs becomes the coefficient for x^3y^5. Based on this, what is the coefficient for x^3y^5?
the coefficient of x^m y^n is C(N,m) where m+n=N
C(8,3) = C(8,5) = 8*7*6/3! = 56