Question

等比数列的公比为2,S4=1,求前8项的和

Answer 1

S4=a1(1-q^4)/(1-q)=a1(1-16)/(1-2)=15a1=1

所以a1=1/15

S8=a1(1-q^8)/(1-q)=1/15*(1-2^8)/(1-2)=17