题目内容：

数列an满足a1=1,a2=2,an+2=(1-1/3cos^2n*π/2)+2sin^2*π/2,（1）求a3、a4及数列an的通项公式（2）设S...
数列an满足a1=1,a2=2,an+2=(1-1/3cos^2n*π/2)+2sin^2*π/2,（1）求a3、a4及数列an的通项公式（2）设Sn＝a1＋a2＋…＋an,求Sn.