A:量入为出适度消费
B:避免盲从讲究实惠
C:精神消费可以取代物质消费
D:避免只重物质消费,忽视精神消费的倾向
—Did you see _______ at the door? There is a boy waiting for you.
—No, I saw_______.
数列an的通项公式an=ncosnπ2,其前n项和为Sn,则S2013等于( )