câu `1:` 

\(c,A=\dfrac{5}{1\cdot3}+\dfrac{5}{3\cdot5}+\dfrac{5}{5\cdot7}+...+\dfrac{5}{2021\cdot2023}\\ \dfrac{2}{5}A=\dfrac{2}{1\cdot3}+\dfrac{2}{3\cdot5}+\dfrac{2}{5\cdot7}+...+\dfrac{2}{2021\cdot2023}\\\dfrac{2}{5}A=1-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}+...+\dfrac{1}{2021}-\dfrac{1}{2023}\\ \dfrac{2}{5}A=1-\dfrac{1}{2023}\\ A=\dfrac{2022}{2023}:\dfrac{2}{5}=\dfrac{5055}{2023} \)

bài 2 :

\(a,x+\dfrac{1}{2}=\dfrac{1}{4}+\dfrac{2}{4}\\ x+\dfrac{1}{2}=\dfrac{3}{4}\\ x=\dfrac{3}{4}-\dfrac{1}{2}\\ x=\dfrac{1}{4}\\ b,814-\left(x-305\right)=712\\ x-305=814-712\\ x-305=102\\ x=102+305\\ x-407\\ c,\left(\dfrac{7}{12}+x\right)-\dfrac{11}{12}=\dfrac{2}{3}\\ \dfrac{7}{12}++x=\dfrac{2}{3}+\dfrac{11}{12}\\ \dfrac{7}{12}+x=\dfrac{19}{12}\\ x=\dfrac{19}{12}-\dfrac{7}{12}=1\)

\(d,\dfrac{232323}{242424}:x=\dfrac{1}{1\cdot4}+\dfrac{1}{4\cdot7}+\dfrac{1}{7\cdot10}+\dfrac{1}{10\cdot13}+\dfrac{1}{13\cdot16}\\ \dfrac{23}{24}:x=\dfrac{1}{1\cdot4}+\dfrac{1}{4\cdot7}+\dfrac{1}{7\cdot10}+\dfrac{1}{10\cdot13}+\dfrac{1}{13\cdot16}\\ \dfrac{23}{24}:3x=\dfrac{3}{1\cdot4}+\dfrac{3}{4\cdot7}+\dfrac{3}{7\cdot10}+\dfrac{3}{10\cdot13}+\dfrac{3}{13\cdot16}\\ \dfrac{23}{24}:3x=1-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{10}+\dfrac{1}{10}-\dfrac{1}{13}+\dfrac{1}{13}-\dfrac{1}{16}\\ \dfrac{23}{24}:3x=\dfrac{15}{16}\\ x=\dfrac{135}{46}\)

._. mik bt mờ  

mik nghĩ là  \(2^3\cdot5\)

\(2^3\cdot5-\left[131-\left(4+2^3\right)^2\right]\\ =2^3\cdot5-\left[131-\left(4+8\right)^2\right]\\ =2^3\cdot5-\left[131-144\right]\\ =2^3\cdot5+13\\ =40+13=53\)

Cạnh : `28/4=7(cm)`

`S=7xx7=49(cm^2)`

\(92\cdot4-27=\left(x+350\right):x+315\\ \Leftrightarrow368-27=\left(x+360\right):x+215\\ \Leftrightarrow341=\left(x+360\right):x+215\\ \Leftrightarrow\left(x+360\right):x=341-215\\ \Leftrightarrow\left(x+360\right):x=126\\ \Leftrightarrow\left(x:x\right)+\left(360:x\right)=126\\ \Leftrightarrow1+360:x=126\\ \Leftrightarrow360:x=125\\ \Leftrightarrow x=2,88\)