等比數(shù)列之和怎么算
2025-08-24
q≠1時(shí)，Sn=a1(1-q^n)/(1-q)=(a1-anq)/(1-q)，q=1時(shí)Sn=na1。(a1為首項(xiàng)，an為第n項(xiàng)，d為公差，q為等比) 證明過程 等比數(shù)列前n項(xiàng)和Sn=a1+a2+.+an=a1(1-q^n)/(1-q) (公比q≠1) 證：Sn=a1+a1q+a1q^2.+a1q^(n-1).(1) qSn=a1q+a1q^2+.a1q^(n-1)+a1q^n.(2)...