Bài 2:

\(B=\frac13+\frac19+\cdots+\frac{1}{729}\)

=>\(3\times B=1+\frac13+\ldots+\frac{1}{243}\)

=>3B-B=\(1+\frac13+\cdots+\frac{1}{243}-\frac13-\frac19-\cdots-\frac{1}{729}\)

=>2B=\(1-\frac{1}{729}=\frac{728}{729}\)

=>\(B=\frac{728}{729}:2=\frac{364}{729}\)

Bài 3: \(C=\frac23+\frac29+\cdots+\frac{2}{729}\)

=>3C=\(2+\frac23+\cdots+\frac{2}{243}\)

=>3C-C=\(2+\frac23+\cdots+\frac{2}{243}-\frac23-\frac29-\cdots-\frac{2}{729}\)

=>2C=\(2-\frac{2}{729}\)

=>\(C=1-\frac{1}{729}=\frac{728}{729}\)

Bài 5:

\(E=\frac12+\frac14+\cdots+\frac{1}{1024}\)

=>2E=\(1+\frac12+\cdots+\frac{1}{512}\)

=>2E-E=\(1+\frac12+\cdots+\frac{1}{512}-\frac12-\frac14-\cdots-\frac{1}{1024}\)

=>E=\(1-\frac{1}{1024}=\frac{1023}{1024}\)

bài 4:

<=> \(2D=\frac{10}{3}+\frac53+\frac56+\cdots+\frac{5}{192}+\frac{5}{384}\)

=> \(2D-D=\left(\frac{10}{3}+\frac53+\frac56+\cdots+\frac{5}{192}+\frac{5}{384}\right)-\left(\frac53+\frac56+\cdots+\frac{5}{768}\right)\)

\(D=\frac{10}{3}-\frac{5}{768}=\frac{2555}{768}\)

Bài 6:

<=> \(3D=\frac{12}{3}+\frac49+\cdots+\frac{4}{81}+\frac{4}{243}\)

=> \(3D-D=\left(\frac{12}{3}+\frac49+\cdots+\frac{4}{81}+\frac{4}{243}\right)-\left(\frac49+\cdots+\frac{4}{729}\right)\)

\(2D=\frac43-\frac{4}{729}=\frac{968}{729}\)

=> \(D=\frac{484}{729}\)

Bài 7:

<=> \(4K=\frac{12}{7}+\frac37+\frac{3}{28}+\frac{3}{112}+\frac{3}{448}\)

=> \(4K-K=\left(\frac{12}{7}+\frac37+\frac{3}{28}+\frac{3}{112}+\frac{3}{448}\right)-\left(\frac37+..+\frac{3}{1792}\right)\)

\(3K=\frac{12}{7}-\frac{3}{1792}\)

\(3K=\frac{3069}{1792}\)

=> \(K=\frac{1023}{1792}\)

Bài 8:

<=> \(2A=\frac65+\frac35+\frac{3}{10}+..+\frac{3}{80}\)

\(2A-A=\left(\frac65+\frac35+\cdots+\frac{3}{80}\right)-\left(\frac35+\frac{3}{10}+\frac{3}{20}+\frac{3}{40}+\frac{3}{80}+\frac{3}{160}\right)\)

\(A=\frac65-\frac{3}{160}=\frac{189}{160}\)

Bài 9:

<=> \(9D=\frac{18}{7}+\frac27+\frac{2}{63}+\frac{2}{567}\)

=> \(9D-D=\left(\frac{18}{7}+\frac27+\frac{2}{63}+\frac{2}{567}\right)-\left(\frac27+\frac{2}{63}+\frac{2}{567}+\frac{2}{5103}\right)\)

\(8D=\frac{18}{7}-\frac{2}{5103}=\frac{13120}{5103}\)

=> \(D=\frac{1640}{5103}\)

sao cong vậy bn tui nhìn ko nổi luôn