Question

S_n=1-2+3-4+…+(-1)^n-1n,则S₁₇+S₃₃+S₅₀等于(    )

A.0               B.1                 C.-1              D.2

Answer 1

B

解析:n为偶数,S_n=1-2+3-4+…+(n-1)-n=,n为奇数,S_n=1-2+3-4+5-…-(n-1)+n

=1+,

所以S₁₇=9,S₃₃=17,S₅₀=-25,故选B.