如图是半圆的直径,是弧的三等分点,是线段的三等分点,若,则 .
已知数列{an}满足a1=1,an+1-2an=2n,则an=_______
已知数列{an}满足a1=1,an+1=,则an=_______
设等差数列{an}的前n项和为Sn,则S4,S8-S4,S12-S8,S16-S12成等差数列.类比以上结论有:设等比数列{bn}的前n项积为Tn,则T4,________,________,成等比数列.
等比数列{an}的公比q>0,已知a2=1,an+2+an+1=6an,则{an}的前4项和S4=_______
函数y=sin的递减区间是________;函数y=lg cos x的递减区间是_______