考研数学无穷级数收敛的选择题一道

http://wenku.baidu.com/link?url=qQNwHd4MEAOw70GKVoafD2JOoQMRD57oIs16f58iairo5b0gizVtK3YgAgzHcGWtZJ_b1oBQCdZRpKCfT8CsV45kSw5qiVMrioZyb-LgyXm |sin(x)|<|x| |nsin(a/n+1)|<a*n/(n+1)<a 所以答案中的不等式成立