解ï¼
æ°åæ¯çå·®æ°åï¼a2+a4=a1+a5=10
â21æ¯a2ãa4ççæ¯ä¸é¡¹ï¼a2·a4=(â21)²=21
a2ãa4çæ¹ç¨x²-10x+21=0çä¸¤æ ¹
(x-3)(x-7)=0
x=3æx=7
æ°å{an}æ¯éå¢æ°åï¼a2=3ï¼a4=7
d=(a4-a2)/2=(7-3)/2=2
a1=a2-d=3-2=1
an=a1+(n-1)d=1+2(n-1)=2n-1
æ°å{an}çé项å
¬å¼ä¸ºan=2n-1
bn=an/2ⁿ=(2n-1)/2ⁿ
Tn=b1+b2+b3+...+bn=1/2 +3/2²+ 5/2³+...+(2n-1)/2ⁿ
½Tn=1/2²+ 3/2³+...+(2n-3)/2ⁿ +(2n-1)/2ⁿ⁺¹
Tn-½Tn=½Tn=1/2 +2/2²+ 2/2³+...+ 2/2ⁿ -(2n-1)/2ⁿ⁺¹
=1/2 +1/2 +1/2²+...+ 1/2ⁿ⁻¹ -(2n-1)/2ⁿ⁺¹
=½+ ½Â·(1-½ⁿ⁻¹)/(1-½) -(2n-1)/2ⁿ⁺¹
=3/2 -(2n+3)/2ⁿ⁺¹
Tn=3- (2n+3)/2ⁿ
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