6. 已知各项均为正数的等比数列 $\left\{a_{n}\right\}$ 的前 4 项和为 15 , 且 $a_{5}=3 a_{3}+4 a_{1}$, 则 $a_{3}=(\quad)$
Gaokao · Math · previous-year question
- A.16
- B.8
- C.4
- D.2
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