一个等差数列的首项为 aa,公差为 dd,其中 aa 和 dd 均非零。如果该等差数列的第一项、第五项和第十一项分别等于一个等比数列的第一项、第二项和第三项,求该等比数列前十项的和的精确值,并表示为 kaka 的形式,其中 kk 是一个最简有理数。

先不看答案,让 AI 诊断你卡在哪里?

它不会直接告诉你答案,而是问你卡在哪一步看看 AI 是怎么陪你一步步弄懂这道题的

完整解法

1

1. Set up an equation by expressing the terms using the arithmetic progression and equating the ratio of consecutive terms for the geometric progression.

Let the terms of the geometric series be T1T_1, T2T_2, and T3T_3. From the arithmetic progression, we have:
T1=aT_1 = a
T2=a+4dT_2 = a + 4d
T3=a+10dT_3 = a + 10d

Since these terms form a geometric series, the ratio between consecutive terms must be equal:
a+4da=a+10da+4d\frac{a + 4d}{a} = \frac{a + 10d}{a + 4d}

2. Solve the equation to find a relationship between $d$ and $a$.

Cross-multiplying to solve for dd:
(a+4d)2=a(a+10d)(a + 4d)^2 = a(a + 10d)
a2+8ad+16d2=a2+10ada^2 + 8ad + 16d^2 = a^2 + 10ad
16d2−2ad=016d^2 - 2ad = 0
2d(8d−a)=02d(8d - a) = 0

Since it is given that d≠0d \neq 0, we can divide by 2d2d (or select the non-zero root):
8d−a=08d - a = 0
d=a8d = \frac{a}{8}

3. Calculate the common ratio $r$ of the geometric series.

Substitute d=a8d = \frac{a}{8} back into the ratio expression:
r=a+4(a8)ar = \frac{a + 4\left(\frac{a}{8}\right)}{a}
r=a+12aar = \frac{a + \frac{1}{2}a}{a}
r=32r = \frac{3}{2}

4. Use the formula for the sum of a finite geometric series to find the sum of the first ten terms.

The sum of the first 10 terms is given by S10=a(r10−1)r−1S_{10} = \frac{a(r^{10} - 1)}{r - 1}:
S10=a((32)10−1)32−1S_{10} = \frac{a\left(\left(\frac{3}{2}\right)^{10} - 1\right)}{\frac{3}{2} - 1}
S10=a(590491024−1)12S_{10} = \frac{a\left(\frac{59049}{1024} - 1\right)}{\frac{1}{2}}
S10=2a(59049−10241024)S_{10} = 2a\left(\frac{59049 - 1024}{1024}\right)
S10=2a(580251024)S_{10} = 2a\left(\frac{58025}{1024}\right)
S10=58025512aS_{10} = \frac{58025}{512}a

答案

58025512a\frac{58025}{512}a

AI 分析,可能出错

用 AI 批改,圈出写错的那一步,并自动将你的薄弱点存入错题本,形成你的错因库。

同一知识点的其他题

  1. 1已知一个数列的前 nn 项和为 Sn=3n(n+2)S_n = 3n(n + 2)。证明该数列是一个等差数列。 第 (a) 部分中数列的第二项、第七项和第 mm 项是一个等比数列的前三个连续项,该等比数列的第 nn 项记为 vnv_n。求 mm…
  2. 2已知一个级数的前 nn 项和为 Sn=3n(n+2)S_n = 3n(n + 2)。证明该级数的各项构成等差数列。 (a) 部分中该级数的第二项、第七项和第 mm 项是一个等比级数的前三个连续项,且其第 nn 项由 vnv_n 给出。求 mm 的值…
  3. 3等比数列{an}\{a_{n}\}中,若a2a_{2}、a4a_{4}是方程2x2−11x+8=02x^{2}-11x+8=0的两根,则a3a_{3}的值为()(\quad) A. 22 B. ±2±2 C. 2\sqrt{2}…
  4. 4一个等差数列的前 100 项之和为 10,000。该数列的第一项、第二项和第五项构成一个等比数列的三个连续项。求该等差数列的首项和非零公差。
  5. 5数列 u1,u2,u3,…u_{1}, u_{2}, u_{3}, \ldots 的前 nn 项和为 SnS_{n},其中 Sn=∑r=1nurS_{n} = \sum_{r=1}^{n} u_{r}。已知 Sn=18−2n+13n−2S_{n} = 18 - \dfrac{2^{n+1}}{3^{n-2}}…