Question

數(shù)列1，，，，…，的前n項(xiàng)和等于(    )

A.S_n=3--                            B.S_n=3-^-1-

C.S_n=3--                            D.S_n=3-n2ⁿ-

Answer 1

A

解析：令S_n=1+++…+,                                            ①

則S_n=++…+.                                              ②

①-②得

∴S_n=1++…+

=1+

=.

∴S_n=3--,故選A.

或者用特殊法.