Question

等差数列{an}知a4=10,且a3,a6,a10成等比数列,求{an}的前20项和s20

Answer 1

2.a6-a2=4d=-8

=>d=-2,a1=30

=>Sn=-n^2+31n

=>n=15或n=16有最大值,S15=s16=240

3.S20/S10=(q^20-1)/(q^10-1)=3

=>q^10=2=p

S30/S20=(q^30-1)/(q^20-1) =(p^3-1)/(p^2-1)=(p^2+p+1)/(1+p)=7/3

S30=7S20/3=56