a) 10 - 2 = 8     10 - 4 = 6     10 - 3 = 7     10 - 7 = 3     10 - 5 = 5

10 - 9 = 1     10 - 6 = 4     10 - 1 = 9     10 - 0 = 10     10 - 10 = 0

   10 + 10 + 10 + 10 + 10 + 10 . 0

= 10 + 10 + 10 + 10 + 10

= 10 . 5

= 50

Hk tốt

A=10 + 10 +10 + 10 + 10 + ( 10.0)

A=10+10+10+10+10+0

A=50

10 + 10 + 10 + 10 + 10 + 10 . 0

=  10 + 10 + 10 + 10 + 10

=   10 . 5

=  50

\(A=\frac{10}{11.16}+\frac{10}{16.21}+...+\frac{10}{61.66}\)

\(A=\frac{10}{5}\left(\frac{5}{11.16}+\frac{5}{16.21}+...+\frac{5}{61.66}\right)\)

\(A=\frac{10}{5}\left(\frac{1}{11}-\frac{1}{16}+\frac{1}{16}-\frac{1}{21}+...+\frac{1}{61}-\frac{1}{66}\right)\)

\(A=\frac{10}{5}\left(\frac{1}{11}-\frac{1}{66}\right)=\frac{10}{5}.\frac{5}{66}=\frac{5}{33}\)

Vậy A =5/33

Ta có:

10A=10+10²+...........+10⁹⁹

10A-A=10⁹⁹+.........+10-1-.........-10⁹⁸

9A=10⁹⁹-1

9A=1000.....00            - 1

        99 chữ số 0

9A=999......999

           99 chữ số 9

A=11111......111

        99 chữ số 1

\(A=1+10+10^2+10^3+....+10^{98}.\)

\(\Rightarrow4A=10+10^2+10^3+10^4+...+10^{99}\)

4A - A = ( 10 + 10² + 10³ + 10⁴ + ... + 10⁹⁹) - ( 1 + 10 + 10²+ 10³+ ......... + 10⁹⁸)

\(\Rightarrow3A=10^{99}-1\)

\(A=\frac{10^{99}-1}{3}\)

\(A=1+10+10^2+.....+10^{98}.\)

\(\Rightarrow10A=10+10^2+10^3+...+10^{99}\)

\(\Rightarrow10A-A=\left(10+10^2+10^3+...+10^{99}\right)-\left(1+10+10^2+.....+10^{98}\right)\)

\(\Rightarrow9A=10^{99}-1\)

\(\Rightarrow A=\frac{100....00-1}{9}\)( 100 chữ số 0)

\(\Rightarrow A=\frac{99...99}{9}\)( 99 chữ số 9)

\(\Rightarrow A=111.....1\)(99 chữ số 1) 

\(A=\frac{10}{56}+\frac{10}{140}+\frac{10}{260}+...+\frac{10}{1400}\)

\(A=\frac{5}{28}+\frac{5}{70}+\frac{5}{130}+...+\frac{5}{700}\)

\(\frac{3A}{5}=\frac{3}{4\times7}+\frac{3}{7\times10}+\frac{3}{10\times13}+...+\frac{3}{25\times28}\)

\(\frac{3A}{5}=\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+\frac{1}{10}-\frac{1}{13}+...+\frac{1}{25}-\frac{1}{28}\)

\(\frac{3A}{5}=\frac{1}{4}-\frac{1}{28}\)

\(\frac{3A}{5}=\frac{3}{14}\)

\(A=\frac{3}{14}\times\frac{5}{3}\)

\(A=\frac{5}{14}\)

@py. Hello, hihi

\(A=\frac{10}{5.7}+\frac{10}{7.9}+\frac{10}{9.11}+...+\frac{10}{33.35}\)

\(\Rightarrow A=10.\left(\frac{1}{5.7}+\frac{1}{7.9}+\frac{1}{9.11}+...+\frac{1}{33.35}\right)\)

Đặt \(B=\frac{1}{5.7}+\frac{1}{7.9}+\frac{1}{9.11}+...+\frac{1}{33.35}\)

\(\Rightarrow2B=\frac{2}{5.7}+\frac{2}{7.9}+\frac{2}{9.11}+...+\frac{2}{33.35}\)

\(\Rightarrow2B=\frac{1}{5}-\frac{1}{35}=\frac{6}{35}\)

\(\Rightarrow B=\frac{3}{35}\)

\(\Rightarrow A=10.\frac{3}{35}=\frac{30}{35}=\frac{6}{7}\)

_Học tốt_

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