1.已知数列{an}为等差数列(1)a1=1,d=4,求a20;(2)a1=6,a8=27,求d;(3)a3=8,a7=32,求d和a13 2.根据下列条件,求等差数列{an)前n项和Sn(1)a1=1,an=19,n=10; (2)a1=100,d=-5,n=20; (3)a1=10,d=2分之1,an=20. 3.已知数列的通项an=5n+2,求s10. 4.等差数列10,9,8,7,.前多少项是0?