a: A=3^2(1^2+2^2+...+10^2)

=9*385

=3465

b: B=2^3(1^3+2^3+...+10^3)

=8*3025

=24200

giải dùm mk vs đi

A=1+3+3²+3³+.....+3²⁰²¹
-->3A=3(1+3+3²+3³+.....+3²⁰²¹)
-->3A=3+3²+3³+...+3²⁰²²
-->3A-A=(3+3²+3³+....3²⁰²²)-(1+3+3²+3³+.....+3²⁰²¹)
-->2A=3²⁰²²-1
-->A=(3²⁰²²-1):2
Vì (3²⁰²²-1):2>(3²⁰²²-1):2
-->A=B
 

\(\dfrac{a}{b}=\dfrac{2}{3}\Rightarrow\dfrac{a}{2}=\dfrac{b}{3};\dfrac{a}{c}=\dfrac{1}{2}\Rightarrow\dfrac{a}{1}=\dfrac{c}{2}\\ \Rightarrow\dfrac{a}{2}=\dfrac{b}{3}=\dfrac{c}{4}\Rightarrow\dfrac{a^3}{8}=\dfrac{b^3}{27}=\dfrac{c^3}{64}\)

Áp dụng tcdtsnb:

\(\dfrac{a^3}{8}=\dfrac{b^3}{27}=\dfrac{c^3}{64}=\dfrac{a^3+b^3+c^3}{8+27+64}=\dfrac{99}{99}=1\\ \Rightarrow\left\{{}\begin{matrix}a^3=8\\b^3=27\\c^3=64\end{matrix}\right.\Rightarrow\left\{{}\begin{matrix}a=2\\b=3\\c=4\end{matrix}\right.\)

\(A=3+3^2+3^3+...+3^{100}\)

\(3A=3^2+3^3+3^4+...+3^{101}\)

->\(3A-A=3^{101}-3\)

->2A+3=3¹⁰¹

^->n=101

ai giải mau giúp mình nha

Thanks

a)

A = 3 + 3² + 3³ + ... + 3⁶

3A = 3² + 3³ + 3⁴ + ...  + 3⁷

3A - A = (3² + 3³ + 3⁴ + ...  + 3⁷) - (3 + 3² + 3³ + ... + 3⁶)

2A = 3⁷ - 3

A = \(\dfrac{3^7-3}{2}\)

b)

Từ câu a) suy ra

2A - 3 = 3^x

3⁷ - 3 - 3 = 3^x (rõ ràng đề sai)

c)

A = 3 + 3² + 3³ + ... + 3⁶

A = 3(1 + 3¹) + 3³(1 + 3¹) + 3⁵(1 + 3¹)

A = (3 + 3³ + 3⁵).4

Do đó A ⋮ 4