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解答 - 几何数列

公比是：

r = 0.7272727272727273

r=0.7272727272727273

该系列的和是：

s = - 19

s=-19

此系列的通用形式是：

a_{n} = - 11 \cdot {0.7272727272727273}^{n - 1}

a_n=-11*0.7272727272727273^(n-1)

这个序列的第n项是：

- 11, - 8, - 5.818181818181818, - 4.231404958677686, - 3.0773854244928627, - 2.238098490540264, - 1.627707993120192, - 1.1837876313601396, - 0.8609364591710107, - 0.626135606669826

-11,-8,-5.818181818181818,-4.231404958677686,-3.0773854244928627,-2.238098490540264,-1.627707993120192,-1.1837876313601396,-0.8609364591710107,-0.626135606669826

查看步骤

逐步解答

1. 找到公比

通过将序列中的任何项除以前一项来找到公比：

$a_{2} a_{1} = - \frac{8}{-} 11 = 0.7272727272727273$

该序列的公比（ $r$ ）保持不变，并且等于两个连续项的商。
$r = 0.7272727272727273$

2. 求和

5 个额外步骤

$s_{n} = a * ((1 - r^{n}) / (1 - r))$

要找到系列的和，将第一项： $a = - 11$ 、公比： $r = 0.7272727272727273$ 和元素数目 $n = 2$ 插入几何级数求和公式：

$s_{2} = - 11 * ((1 - {0.7272727272727273}^{2}) / (1 - 0.7272727272727273))$

$s_{2} = - 11 * ((1 - 0.5289256198347108) / (1 - 0.7272727272727273))$

$s_{2} = - 11 * (0.47107438016528924 / (1 - 0.7272727272727273))$

$s_{2} = - 11 * (0.47107438016528924 / 0.2727272727272727)$

$s_{2} = - 11 \cdot 1.7272727272727273$

$s_{2} = - 19$

3. 找到通用形式

$a_{n} = a \cdot r^{n - 1}$

要找到系列的通用形式，将第一项： $a = - 11$ 和公比： $r = 0.7272727272727273$ 插入几何级数的公式：

$a_{n} = - 11 \cdot {0.7272727272727273}^{n - 1}$

4. 找到第n项

使用通用公式找到第n项

$a_{1} = - 11$

$a_{2} = a_{1} \cdot r^{n - 1} = - 11 \cdot {0.7272727272727273}^{2 - 1} = - 11 \cdot {0.7272727272727273}^{1} = - 11 \cdot 0.7272727272727273 = - 8$

$a_{3} = a_{1} \cdot r^{n - 1} = - 11 \cdot {0.7272727272727273}^{3 - 1} = - 11 \cdot {0.7272727272727273}^{2} = - 11 \cdot 0.5289256198347108 = - 5.818181818181818$

$a_{4} = a_{1} \cdot r^{n - 1} = - 11 \cdot {0.7272727272727273}^{4 - 1} = - 11 \cdot {0.7272727272727273}^{3} = - 11 \cdot 0.38467317806160783 = - 4.231404958677686$

$a_{5} = a_{1} \cdot r^{n - 1} = - 11 \cdot {0.7272727272727273}^{5 - 1} = - 11 \cdot {0.7272727272727273}^{4} = - 11 \cdot 0.279762311317533 = - 3.0773854244928627$

$a_{6} = a_{1} \cdot r^{n - 1} = - 11 \cdot {0.7272727272727273}^{6 - 1} = - 11 \cdot {0.7272727272727273}^{5} = - 11 \cdot 0.20346349914002398 = - 2.238098490540264$

$a_{7} = a_{1} \cdot r^{n - 1} = - 11 \cdot {0.7272727272727273}^{7 - 1} = - 11 \cdot {0.7272727272727273}^{6} = - 11 \cdot 0.14797345392001746 = - 1.627707993120192$

$a_{8} = a_{1} \cdot r^{n - 1} = - 11 \cdot {0.7272727272727273}^{8 - 1} = - 11 \cdot {0.7272727272727273}^{7} = - 11 \cdot 0.10761705739637634 = - 1.1837876313601396$

$a_{9} = a_{1} \cdot r^{n - 1} = - 11 \cdot {0.7272727272727273}^{9 - 1} = - 11 \cdot {0.7272727272727273}^{8} = - 11 \cdot 0.07826695083372824 = - 0.8609364591710107$

$a_{10} = a_{1} \cdot r^{n - 1} = - 11 \cdot {0.7272727272727273}^{10 - 1} = - 11 \cdot {0.7272727272727273}^{9} = - 11 \cdot 0.056921418788166 = - 0.626135606669826$

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几何数列