1 已知数列1, 3 {\displaystyle {\sqrt {3}}} , 5 {\displaystyle {\sqrt {5}}} ,……, 2 n − 1 {\displaystyle {\sqrt {2n-1}}} ,……,
则 21 {\displaystyle {\sqrt {21}}} 是这个数列的
2 在等差数列{ a n {\displaystyle a_{n}} }中,已知 a 5 {\displaystyle a_{5}} =3, a 9 {\displaystyle a_{9}} =6,
则a13=?
3 已知数列{an},a≠0,若a1=3,2an+1-an=0,
则a6=