Ta có :

A=2+2²⁺....+2²⁰

A=2\(\left(1+2\right)\)+2²\(\left(1+2\right)\)....+2¹⁹\(\left(1+2\right)\)

A=2.3+2².3+......+2¹⁹.3

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

A chia hết cho 3

\(\left(2+1\right)\)*x=\(\frac{54}{7}\)

3x=\(\frac{54}{7}\)

x=\(\frac{54}{7}\):3

x=\(\frac{18}{7}\)

D=\(\frac{1}{1^2}\)-\(\frac{1}{2^2}\)+\(\frac{1}{2^2}\)-\(\frac{1}{3^2}\)+...+\(\frac{1}{9^2}\)-\(\frac{1}{10^2}\)

D=\(\frac{1}{1^2}\)-\(\frac{1}{10^2}\)

D=\(1\)-\(\frac{1}{100}\)

D=\(\frac{99}{100}\)