| a5 |
| a2 |
∴q=3
∴a1=
| a2 |
| q |
∴S4=
| a1(1?q4) |
| 1?q |
故答案為120
等比數(shù)列{an}中，a2=9，a5=243，則{an}的前4項(xiàng)和為 _．
等比數(shù)列{an}中，a2=9，a5=243，則{an}的前4項(xiàng)和為 ______．
數(shù)學(xué)人氣：432 ℃時(shí)間：2020-06-21 13:18:02
優(yōu)質(zhì)解答
q3=
=27
∴q=3
∴a1=
=3
∴S4=
=120
故答案為120
∴q=3
∴a1=
∴S4=
故答案為120
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