已知数列{an}满足a1=1,a2=a(a>0),数列{bn}=an*an+

问题描述:

已知数列{an}满足a1=1,a2=a(a>0),数列{bn}=an*an+
bn=an*an+1 若{an}是等差数列,且b3=12,求a及an通项⑵若{an}是等比数列,求{bn}的前n项的和sn

(1){an}是等差数列,a1=1,a2=a(a>0),an=1+(n-1)(a-1)a3=2a-1,a4=3a-2b3=a3*a4=(2a-1)(3a-2)=12a=2,或-5/6(舍去)所以a=2,an=n(2){an}是等比数列,a1=1,a2=a(a>0),a3=a^2,a4=a^3b3=a3*a4=a^5=12a=5次根号12an=(5次...