Question

在()ⁿ的二项展开式中,若常数项为60,则n等于(    )

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

Answer 1

答案:B

解析:设常数项为T_r+1==60.

∴                                                ①

∵为非负整数,∴r=0,1,2.

当r=0时,①式左边=1,右边=60,左≠右.(舍去)

当r=1时,①式左边=3,右边=30,左≠右.(舍去)

当r=2时,①式左边=15,右边=15,左=右.故选B.