(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