(1) a n=2n-1 b n=2 n-(2) S n=6-
(1)设{a n}的公差为d,{b n}的公比为q,则依题意有q>0且
解得
所以a n="1+(n-1)" d=2n-1,b n=q n-1=2 n-1.
(2)
=
,
S n=1+
+
+…+
+
, ①
2S n=2+3+
+…+
+
. ②
②-①,得S n=2+2+
+
+…+
-
=2+2×(1+
+
+…+
)-
,
=2+2×
-
=6-
.
设{a n }是等差数列,{b n }是各项都为正数的等比数列,且a 1 =b 1 =1,a 3 +b 5 =21,a
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