Question

Bài 1. Tính M = 3 + 3² + 3³ + ... + 3 mũ8

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Answer 1

M=3+3^2+...+3^8  (1)

3M=3^2+3^3+...+3^9  (2)

tru ve voi ve (2)cho (1) ta co:

3M-M=(3^2+3^3+...+3^9)-(3+3^2+...+3^8)

2M=3^9-3

M=(3^9-3):2

Answer 2

=> 3M = 3²+3³+3⁴+......+3⁹

3M-M=3⁹-3

2M=3⁹-3

M=​​(3⁹-3)/2

Answer 3

\(M=3+3^2+3^2+3^3+...+3^8\)

\(\Rightarrow3M=3\left(3+3^2+3^3+...+3^8\right)\)

\(\Rightarrow3M=3^2+3^3+3^4+...3^9\)

\(\Rightarrow3M-M=\left(3^2+3^3+3^4+...+3^9\right)-\left(3+3^2+3^3+...+3^8\right)\)

\(\Rightarrow2M=3^9-3\)

\(\Rightarrow M=\frac{3^9-3}{2}\)

Vậy \(M=\frac{3^9-3}{2}\)