Question

在()ⁿ的二項展開式中,若常數(shù)項為60,則n等于(    )

A.3              B.6                 C.9               D.12

Answer 1

答案:B

解析:設常數(shù)項為T_r+1==60.

∴                                                ①

∵為非負整數(shù),∴r=0,1,2.

當r=0時,①式左邊=1,右邊=60,左≠右.(舍去)

當r=1時,①式左邊=3,右邊=30,左≠右.(舍去)

當r=2時,①式左邊=15,右邊=15,左=右.故選B.