等比數列的前n項和S[n]=k*3^n+1,求k的值（ ）A.全體實數B.-1C.1D.3

熱心網友

等比數列的前n項和S[n]=k*3^n+1,求k的值解:a1=s1=3k+1 a2=s2-s1=(9k+1)-(3k+1)=6k a3=s3-s2=(27k+1)-(9k+1)=18k a3/a2=a2/a1,18k/6k=6k/(3k+1) 36k^2=54k^2+18k 18k^2+18k=0 k^2+k=0 k(k+1)=0,k=0(不合),k=-1,選B

熱心網友

B