(xi) (33)+(43)+(53)+⋯+(n+23)=(n+34)\dbinom{3}{3} + \dbinom{4}{3} + \dbinom{5}{3} + \cdots + \dbinom{n+2}{3} = \dbinom{n+3}{4}(33)+(34)+(35)+⋯+(3n+2)=(4n+3)