Arithmetic/Geometric Sequence, Series, and Progression 等差/等比的數列/級數

等差數列/算數數列 arithmetic sequence/arithmetic progression 1,3,5,7...
等差級數/算數級數 arithmetic series 1+3+5+7+...
等差級數和：

n(a₁ + a_n)/2

1+3+5+7+9 = 5×(1+9)/2 = 25

等比數列/幾合數列 geometric sequence/geometric progression 1,2,4,8...
等比級數/幾合級數 geometric series 1+2+4+8+...

等比級數和：

a(1-rⁿ)/(1-r) = a(rⁿ-1)/(r-1)

1+2+4+8+16 = 1×(1-2⁵)/(1-2) = 1×(2⁵-1)/(2-1) = 1×(32-1)/(2-1) = 31

公差 common difference
公比 common ratio

參考資料

https://en.wikipedia.org/wiki/Arithmetic_progression
https://en.wikipedia.org/wiki/Geometric_progression
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