Question

1.已知数列{a n}的前n项和为Sn,且Sn=2（a n－1）,则a2等于.

Answer 1

(1)s1=a1=2a1-2

a1=2

s2=a1+a2=2a2-2

a2=4

(2)an=sn-sn-1=5*n－3-[5*(n-1)－3]=5 (n大于1) a1=2(n=1)

3.在公差不为零的等差数列{a n]中,a1,a2为方程X*2－a3X+a4=0的两实数根,求此数列的通项公式.题意得 a1+a2=a3 a1*a2=a4

a1+a1+d=a1+2d a1=d a1*(a1+d)=(a1+3d) d*2d=4d d=2 a1=2

an=2+(n-1)*2=2n