Corollary4.5:Lets={n;°a1n2°a2,…,nka13}, and n > r for each i=1.2 Then the number of r-combinations of s is C(k+r-1 r) Example Suppose that a cookie shop has seven different kinds of cookies. How many dififerent ways can twelve cookies be chosen?▪ Corollary 4.5: Let S={n1 •a1 ,n2 •a2 ,…,nk •ak }， and ni r for each i=1,2,…,n. Then the number of r-combinations of S is C(k+r-1,r). ▪ Example Suppose that a cookie shop has seven different kinds of cookies. How many different ways can twelve cookies be chosen?