Question

已知等比数列｛an｝的前3项和为9/2,前6项的和为14/3

Answer 1

a1(1-q^3)/(1-q)=9/2

a1(1-q^6)/(1-q)=14/3

(1-q^3)/(1-q^6)=27/28

1-q^6=(1-q^3)(1+q^3)

1/(1+q^3)=27/28

1+q^3=28/27

q^3=1/27

q=1/3

a1=(9/2)*(1-1/3)/(1-1/27)=3/(1-1/27)=81/26

追加分20吧