Question

已知数列an中,前n项和Sn满足Sn=2n的2次方+3n-4,求an

Answer 1

n=1,a1=s1=2+3-4=1

Sn=2n^2+3n-4 (1)

Sn-1=2(n-1)^2+3(n-1)-4 ,n≥2 (2)

(1)-(2),得

Sn-Sn-1=2n^2+3n-4-2(n-1)^2-3(n-1)+4

即 an=4n+1 ,n≥2

所以 1 (n=1)

an =

4n+1 (n≥2)