ta gọi \(\frac{1}{2}+\frac{1}{6}+...+\frac{1}{90}\)là A

\(A=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{9.10}\)

\(\Leftrightarrow1.\left(\frac{1}{1}-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+....+\frac{1}{9}-\frac{1}{10}\right)\)

\(\Rightarrow A=1-\frac{1}{10}=\frac{9}{10}\)

ta gọi B là biểu thức thứ2

\(B=\frac{2.2}{3}\times\frac{3.3}{2.4}\times\frac{4.4}{3.5}\times...\times\frac{10.10}{9.11}\)

\(\Rightarrow\)2 x \(\frac{10}{11}\)\(=\frac{20}{11}\)

\(\Rightarrow\)\(x+\frac{9}{10}=\frac{20}{11}+\frac{9}{110}\)

\(\Rightarrow x=1\)

mk nghĩ vậy bạn ạ, mk mong nó đúng

Cách giải

X = 8/9 / ( 1+1/3+1/6+1/10+1/15+1/21)

1+1/3=4/3

4/3+1/6=3/2

3/2+1/10=8/5

8/5+1/15=5/3

5/3+1/21=12/7

4/3+3/2=17/6

8/5+5/3=49/15

(17/6+49/15)+17/6=183/30+17/6=286/30

8/9:286/30=2288/270=1140/135=228/27=76/9

X = 76/9

a) x = 99/20

b) x = 7

c) x = 2

( chỉ lm đc đến đó thui nk )

\(a,\)\(-\frac{3}{5}\cdot x=\frac{1}{4}+0,75\)

\(-\frac{3}{5}\cdot x=\frac{1}{4}+\frac{3}{4}=\frac{4}{4}=1\)

\(x=1\div\left(-\frac{3}{5}\right)\)

\(x=-\frac{5}{3}\)

\(b,\)\(\left(\frac{1}{7}-\frac{1}{3}\right)\cdot x=\frac{28}{5}\times\left(\frac{1}{4}-\frac{1}{7}\right)\)

\(\left(\frac{3}{21}-\frac{7}{21}\right)\cdot x=\frac{28}{5}\cdot\left(\frac{7}{28}-\frac{4}{28}\right)\)

\(-\frac{4}{21}\cdot x=\frac{28}{5}\cdot\frac{3}{28}\)

\(-\frac{4}{21}\cdot x=\frac{3}{5}\)

\(x=\frac{3}{5}\div\left(-\frac{4}{21}\right)\)

\(x=-\frac{63}{20}\)

\(c,\)\(\frac{5}{7}\cdot x=\frac{9}{8}-0,125\)

\(\frac{5}{7}\cdot x=\frac{9}{8}-\frac{1}{8}\)

\(\frac{5}{7}\cdot x=1\)

\(x=1\div\frac{5}{7}\)

\(x=\frac{7}{5}\)

\(d,\)\(\left(\frac{2}{11}+\frac{1}{3}\right)\cdot x=\left(\frac{1}{7}-\frac{1}{8}\right)\cdot36\)

\(\left(\frac{6}{33}+\frac{11}{33}\right)\cdot x=\left(\frac{8}{56}-\frac{7}{56}\right)\cdot36\)

\(\frac{17}{33}\cdot x=\frac{1}{56}\cdot36\)

\(\frac{17}{33}\cdot x=\frac{9}{14}\)

\(x=\frac{9}{14}\div\frac{17}{33}\)

\(x=\frac{9}{14}\cdot\frac{33}{17}=\frac{297}{238}\)

a)\(\frac{3}{5}\times x=1\)                                        

\(x=1\div\frac{3}{5}\)

\(x=\frac{5}{3}\)

Mình chỉ làm duoc câu a thôi 

bằng 2

đáp án đúng = 2

ms làm hôm nay xong

\(\frac{4}{7}\times x=\frac{1}{5}+\frac{2}{3}\)

\(\frac{4}{7}x=\frac{13}{15}\)

\(\Rightarrow x=\frac{91}{60}\)

các bài còn lại tương tự nha 

mấy cái này dễ mà toán tìm x này là cơ bản!!

67865785685685785785774677567568568

\(\frac{4}{7}\times x-\frac{2}{3}=\frac{1}{5}\)

\(\frac{4}{7}\times x=\frac{2}{3}+\frac{1}{5}\)

\(\frac{4}{7}\times x=\frac{13}{15}\)

\(x=\frac{13}{15}\div\frac{4}{7}\)

\(x=\frac{91}{60}\)

a) \(x-\frac{10}{3}=\frac{7}{15}\cdot\frac{3}{5}\)                          b) \(x+\frac{3}{22}=\frac{27}{121}\cdot\frac{11}{9}\)

\(\Leftrightarrow x-\frac{10}{3}=\frac{7}{25}\)                                \(\Leftrightarrow x+\frac{3}{22}=\frac{3}{11}\)

\(\Rightarrow x=\frac{7}{25}+\frac{10}{3}\)                                   \(\Rightarrow x=\frac{3}{11}-\frac{3}{22}\)

       \(x=\frac{271}{75}\)                                                  \(x=\frac{3}{22}\)

c) \(\frac{8}{23}.\frac{46}{24}-x=\frac{1}{3}\)                    d) \(1-x=\frac{49}{65}.\frac{5}{7}\)

        \(\Leftrightarrow\frac{2}{3}-x=\frac{1}{3}\)                      \(\Leftrightarrow1-x=\frac{7}{13}\)

                      \(\Rightarrow x=\frac{2}{3}-\frac{1}{3}\)                       \(\Rightarrow x=1-\frac{7}{13}\)

                           \(x=\frac{1}{3}\)                                         \(x=\frac{6}{13}\)

\(\frac{1.2.3....31}{2^{30}.\left(2.3....31\right).32}=\frac{1}{2^{31}.32}=\frac{1}{2^{36}}=2^{-36}=2^x\)

Vậy x=-36

Hok tốt

Câu 1.

ta có: 63/72 =7/8

=>x +3 =7

=>x=...

Câu 2.

=>x +4<11

=>x=0;1;2;3;4;5;6

=>x +4>9

=>x=6;7;8;9;...

=> x=6

Câu 3 như câu 2