tính tổng á :

\(A=3+3^2+3^3+3^4+...+3^{20}.\)

\(\Rightarrow3A=3^2+3^3+3^4+...3^{20}+3^{21}\)

\(\Rightarrow3A-A=\left(3^2+3^3+..+3^{21}\right)-\left(3+3^2+....+3^{20}\right)\)

\(\Rightarrow2A=3^{21}-1\)

\(\Rightarrow A=\frac{3^{21}-1}{2}\)

Chứng tỏ bạn nhé 

Dung minh ch k

(1+2+3+4+5+6+7+8+9+10+11+12+13+14+15+16+17+18+19+20)^2-1

nhưng tại sao lại ra được phép tính như vậy

A=3+3/2+3/2²+...+3/2^a  nên:

=> 2A = 6+3+3/2+3/2² +...+3/2^a-1

=> A= 6 - 3/2^a  ( lấy 2A -A )

Vậy A=6-3/2^a

THANK

A = 1+3+3²+3³+....+3²⁰

3A = 3+3²+3³+3⁴+.....+3²¹

2A = 3A - A = 3²¹ - 1

=> A = \(\frac{3^{21}-1}{2}\)

\(M=1+\frac{1}{3}-\frac{1}{3^2}+\frac{1}{3^3}-\frac{1}{3^4}+......+\frac{1}{3^{19}}-\frac{1}{3^{20}}\)

\(\Rightarrow\frac{1}{3}M=\frac{1}{3}+\frac{1}{3^2}-\frac{1}{3^3}+\frac{1}{3^4}-\frac{1}{3^5}+.......+\frac{1}{3^{20}}-\frac{1}{3^{21}}\)

\(\Rightarrow\frac{1}{3}M+M=1+\frac{1}{3}+\frac{1}{3}-\frac{1}{3^{21}}\)

\(\Rightarrow\frac{4}{3}M=\frac{5}{3}-\frac{1}{3^{21}}\)\(\Rightarrow M=\frac{\frac{5}{3}-\frac{1}{3^{31}}}{\frac{4}{3}}\)

\(A=3+3^2+3^3+...+3^{20}\)

\(A=\left(3+3^2\right)+\left(3^3+3^4\right)+...+\left(3^{19}+3^{20}\right)\)

\(A=3\left(1+3\right)+3^3\left(3+1\right)+...+3^{19}\left(1+3\right)\)

\(\Rightarrow A=4\left(3+3^3+...+3^{19}\right)\)

\(\Rightarrow A⋮4\)

\(A=3+3^2+3^3+...+3^{20}\)

\(\Rightarrow3A=3^2+3^3+...+3^{20}+3^{21}\)

\(\Rightarrow3A-A=\left(3^2+3^3+...+3^{21}\right)-\left(3+3^2+....+3^{20}\right)\)

\(\Rightarrow2A=3^{21}-3\)

\(\Rightarrow A=\frac{3^{21}-3}{2}\)

mình nghĩ = -2525

ai biet giai nhanh ma ro ra nhe

12.(x-1)=64+8

12.(x-1)=72

     x-1=72:12

     x-1=6

         x=6+1

         x=7

học tốt...

12 . ( x - 1 ) : 3 = 4³ + 2³

12 . ( x - 1 ) : 3 = 64 + 8

12 . ( x - 1 ) : 3 = 72

12 . ( x - 1 ) = 72 . 3

12 . ( x - 1 ) = 216

x - 1 = 216 : 12

x - 1 = 18

x = 18 + 1

x = 19