Students have (wear) ... uniform when they go to school.

2

42

D

\(A=2+2^2+2^3+...+2^{2020}\)

\(2A=\left(2+2^2+2^3+..+2^{2020}\right).2\)

\(2A=2^2+2^3+2^4+...+2^{2021}\)

\(2A-A=\left(2^2+2^3+2^4+...+2^{2021}\right)-\left(2+2^2+2^3+...+2^{2020}\right)\)

\(A=2^2+2^3+2^4+...+2^{2021}-2-2^2-2^3-...-2^{2020}\)

\(A=\left(2^{2021}-2\right)+\left(2^2-2^2\right)+\left(2^3-2^3\right)+\left(2^4-2^4\right)+...\left(2^{2020}-2^{2020}\right)\)

\(A=2^{2021}-2+0+0+0+...+0\)

\(A=2^{2021}-2\) (1)

Thay (1) vào 2(A+2)=\(2^x\) ta được:

\(2\left(2^{2021}-2+2\right)=2^x\)

\(2\left(2^{2021+0}\right)=2^x\)

\(2.2^{2021}=2^x\)

\(2^{2022}=2^x\)

\(\Rightarrow\)x=2022

\(\dfrac{1-4x}{3}=\dfrac{14}{18}-\dfrac{x+5}{6}\Rightarrow\dfrac{1-4x}{3}+\dfrac{x+5}{6}=\dfrac{14}{18}\Rightarrow\dfrac{2-8x}{6}+\dfrac{x+5}{6}=\dfrac{7}{9}\Rightarrow\dfrac{2-8x+x+5}{6}=\dfrac{7}{9}\Rightarrow\dfrac{7-7x}{6}=\dfrac{7}{9}\Rightarrow9\left(7-7x\right)=6.7\Rightarrow63-63x=42\Rightarrow63x=63-42\Rightarrow63x=21\Rightarrow x=21:63\Rightarrow x=\dfrac{1}{3}\)