已知数列an中,前n项和Sn满足Sn=2n的2次方+3n-4,求an

问题描述:

已知数列an中,前n项和Sn满足Sn=2n的2次方+3n-4,求an

n=1,a1=s1=2+3-4=1Sn=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)...Sn-Sn-1?????Sn-Sn-1=an