Question

求（a-b)的7次方展开式的第4项

Answer 1

1/2/1
1/3//3/1
1/4/6/4/1
1/5/10/10/5/1
1/6/15/20/15/6/1
1/7/21/35/35/21/7/1
（a-b)的7次方展开式的第k项=35a^(7-k+1)*(-b)^(k-1)
即,-35a^4b^3.