22. (12分)在求$1 + 3 + 3^{2}+3^{3}+3^{4}+3^{5}+3^{6}+3^{7}+3^{8}$的值时,小明发现,从第二个加数起每一个加数都是前一个加数的$3$倍,于是想出了下面的解题方法:
解:设$S = 1 + 3 + 3^{2}+3^{3}+3^{4}+3^{5}+3^{6}+3^{7}+3^{8}$,①
①式的两边都乘以$3$,得$3S = 3 + 3^{2}+3^{3}+3^{4}+3^{5}+3^{6}+3^{7}+3^{8}+3^{9}$.②
由② - ①得$3S - S = 3^{9}-1$,即$2S = 3^{9}-1$,
$\therefore S= \frac{3^{9}-1}{2}$.
请你用此方法解决下列问题,并写出完整的解题步骤.
计算:(1)$1 + 5 + 5^{2}+5^{3}+… + 5^{100}$;
(2)$1 + m + m^{2}+m^{3}+m^{4}+… + m^{2018}(m\neq0且m\neq1)$.
解:设$S = 1 + 3 + 3^{2}+3^{3}+3^{4}+3^{5}+3^{6}+3^{7}+3^{8}$,①
①式的两边都乘以$3$,得$3S = 3 + 3^{2}+3^{3}+3^{4}+3^{5}+3^{6}+3^{7}+3^{8}+3^{9}$.②
由② - ①得$3S - S = 3^{9}-1$,即$2S = 3^{9}-1$,
$\therefore S= \frac{3^{9}-1}{2}$.
请你用此方法解决下列问题,并写出完整的解题步骤.
计算:(1)$1 + 5 + 5^{2}+5^{3}+… + 5^{100}$;
(2)$1 + m + m^{2}+m^{3}+m^{4}+… + m^{2018}(m\neq0且m\neq1)$.
答案
(1)设$S = 1 + 5 + 5^{2} + 5^{3} + \dots + 5^{100}$,①
①式两边都乘以$5$,得$5S = 5 + 5^{2} + 5^{3} + \dots + 5^{100} + 5^{101}$,②
② - ①得$5S - S = 5^{101} - 1$,即$4S = 5^{101} - 1$,
$\therefore S = \frac{5^{101} - 1}{4}$。
(2)设$S = 1 + m + m^{2} + m^{3} + m^{4} + \dots + m^{2018}$,①
①式两边都乘以$m$,得$mS = m + m^{2} + m^{3} + m^{4} + \dots + m^{2018} + m^{2019}$,②
② - ①得$mS - S = m^{2019} - 1$,即$(m - 1)S = m^{2019} - 1$,
$\because m \neq 1$,$\therefore S = \frac{m^{2019} - 1}{m - 1}$。
①式两边都乘以$5$,得$5S = 5 + 5^{2} + 5^{3} + \dots + 5^{100} + 5^{101}$,②
② - ①得$5S - S = 5^{101} - 1$,即$4S = 5^{101} - 1$,
$\therefore S = \frac{5^{101} - 1}{4}$。
(2)设$S = 1 + m + m^{2} + m^{3} + m^{4} + \dots + m^{2018}$,①
①式两边都乘以$m$,得$mS = m + m^{2} + m^{3} + m^{4} + \dots + m^{2018} + m^{2019}$,②
② - ①得$mS - S = m^{2019} - 1$,即$(m - 1)S = m^{2019} - 1$,
$\because m \neq 1$,$\therefore S = \frac{m^{2019} - 1}{m - 1}$。
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