Question

求高手解答数学题

1.已知数列{an}的前n项和Sn满足Sn=n²+1。则an=?
2.已知等差数列a.b.1.c.3. 。。。则该数列的第2009项是？

Answer 1

1,an=Sn-S(n-1)
=n²+1-[(n-1)²+1]
=2n-1
2,等差数列的通项an=a1+(n-1)d
a3=a1+2d=1
a5=a1+4d=3
解得a1=-1; d=1
a2009=a1+(2009-1)d
=2007

Answer 2

1.an=sn-s(n-1)=n^2-1-[(n-1)^2-1]=2n-1
2.a3=a1+2d=1,a5=a1+4d=3,可求得d=1,a1=a=-1
a2009=a1+2008d=-1+2008=2007