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

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

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

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

\(\Rightarrow A⋮4\)

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

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

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

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

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

A=(8+3+4)-(2/7+4/9-1)

A=15 5/9

A=3 1/5

\(\left(1+\frac{1}{2}+\frac{1}{3}+\frac{1}{4}\right)+\left(\frac{1}{2}+\frac{3}{4}+\frac{3}{4}\right)-2=?\)

what?

\(\frac{3}{10}\)x \((\frac{-5}{9}+\frac{3}{5})\)\(+\frac{3}{10}x\)\((\frac{-4}{9}+\frac{2}{5})\)= \(\frac{3}{10}x\)\((\frac{-2}{45})\)+ \(\frac{3}{10}x\) \(\times\frac{2}{45}\)= \(\frac{3}{10}x\left(\frac{-2}{45}+\frac{2}{45}\right)\)= \(\frac{3}{10}x\times0\)= 0

Chúc bạn học tốt !

CỨU MÌNH VỚI !!!!!!!!!!!!!!

\(A=2^0+\left(2+2^2\right)+\left(2^3+2^4\right)+...+\left(2^{99}+2^{100}\right)\)

\(A=1+2\left(1+2\right)+2^3\left(1+2\right)+...+2^{99}\left(1+2\right)\)

\(A=1+3\left(2+2^3+2^5+...+2^{99}\right)\)

A chia 3 dư 1

a) =3/11(-5/12+-7/12)-4/15

= 3/11.(-1)-4/15

=(-3)/11-4/15

=-89/165

a)

\(=\frac{3}{11}\left(\frac{-5}{12}+\frac{-7}{12}\right)-\frac{4}{15}\)

\(=\frac{3}{11}\left(\frac{-12}{12}\right)-\frac{4}{15}\)

\(=\frac{3}{11}\left(-1\right)-\frac{4}{15}\)

\(=\frac{3}{11}-\frac{4}{15}\)

\(=\frac{-89}{165}.\)