刚刚忘记上传word附件了，现在请看，并帮忙解答，写出步骤！提问指数：90
热心网友
an＝1+ =(1-(1/2) n )/(1-1/2)=2 1-n –2an+1- an ＝2 -n –2-2 1-n +2=2 -n-2 1-n =-2 –n乱了看附件吧
热心网友
a1=1; & an=1+1/2+1/4+......+1/2^(n-1)=(1-1/2^n)/(1-1/2)=2-1/2^(n-1) n=2Sn=1+(2-1)+(2-1/2)+(2-1/4)+......+[2-1/2^(n-1)]=1+2(n-1)-[1+1/2+1/4+......+1/2^(n-1)]=2n-1+[2-1/2^(n-1)]=2n+1-1/2^(n-1) (n=2)& S1=1