Gọi số hạng thứ nhất là a thì số hạng thứ hai là \(\dfrac{2}{5}\cdot a\), số hạng thứ ba là \(\dfrac{2}{5}\cdot a+\dfrac{7}{4}\), số hạng thứ tư là \(\dfrac{1}{5}\cdot a\).

Khi đó, ta được:

\(a+\dfrac{2}{5}\cdot a+\dfrac{2}{5}\cdot a+\dfrac{7}{4}+\dfrac{1}{5}\cdot a=\dfrac{57}{4}\\ \Leftrightarrow2a+\dfrac{7}{4}=\dfrac{57}{4}\\ \Leftrightarrow2a=\dfrac{50}{4}\\ \Leftrightarrow a=\dfrac{25}{4}\)

Vậy số hạng thứ nhất là \(\dfrac{25}{4}\), số hạng thứ hai là \(\dfrac{5}{2}\), số hạng thứ ba là \(\dfrac{17}{4}\), số hạng thứ tư là \(\dfrac{5}{4}\).

Ta được: \(14\dfrac{1}{4}=\dfrac{25}{4}+\dfrac{5}{2}+\dfrac{17}{4}+\dfrac{5}{4}\)

Gọi 17 số đó là \(a_1,a_2,...,a_{17}\left(a_i\inℚ,i=\overline{1,17}\right)\)

Theo đề bài, ta có:

\(a_1=a_2^3+a_3^3+a_4^3+...+a_{17}^3\)

\(a_2=a_1^3+a_3^3+a_4^3+...+a_{17}^3\)

Trừ theo vế 2 hệ thức này, ta được:

\(a_1-a_2=a_2^3-a_1^3\)

\(\Leftrightarrow a_1-a_2+a_1^3-a_2^3=0\)

\(\Leftrightarrow\left(a_1-a_2\right)\left[\left(a_1-a_2\right)^2+1\right]=0\)

\(\Leftrightarrow\left[{}\begin{matrix}a_1=a_2\\\left(a_1-a_2\right)^2+1=0\left(loại\right)\end{matrix}\right.\)

Như vậy ta có \(a_1=a_2\)

Chứng minh tương tự, ta thu được \(a_1=a_2=...=a_{17}\)

Thế vào hệ thức đầu tiên trong 2 hệ thức trên, ta có:

 \(a_1=17a_1^3\) 

\(\Leftrightarrow a_1\left(17a_1^2-1\right)=0\)

\(\Leftrightarrow\left[{}\begin{matrix}a_1=0\\a_1=\dfrac{\sqrt{17}}{17}\left(loạivìa_1\inℚ\right)\end{matrix}\right.\)

 Vậy \(\left(a_1,a_2,...,a_{17}\right)=\left(0,0,...,0\right)\) là bộ 17 số duy nhất thỏa mãn ycbt.

\(\dfrac{x-2y}{z-y}=-5\Rightarrow\dfrac{x-2y}{y-z}=5\\ \Rightarrow x-2y=5\left(y-z\right)\\ \Rightarrow x-2y=5y-5z\\ \Rightarrow x+5z=7y\)

Ta có: 

\(\dfrac{1}{7}\cdot\dfrac{x-2z}{y-z}=\dfrac{x-2z}{7\left(y-z\right)}=\dfrac{x-2z}{7y-7z}\\ =\dfrac{x-2z}{x+5z-7z}=\dfrac{x-2z}{x-2z}=1\)

\(\Rightarrow\dfrac{x-2z}{y-z}=1:\dfrac{1}{7}=7\)

1) \(x^2-4=x^2-2^2=\left(x-2\right)\left(x+2\right)\)

2) \(1-4x^2=1-\left(2x\right)^2=\left(1-2x\right)\left(1+2x\right)\)

3) \(4x^2-9=\left(2x\right)^2-3^2=\left(2x-3\right)\left(2x+3\right)\)

4) \(9-25x^2=3^2-\left(5x\right)^2=\left(3-5x\right)\left(3+5x\right)\)

5) \(4x^2-25=\left(2x\right)^2-5^2=\left(2x-5\right)\left(2x+5\right)\)

6) \(9x^2-36=\left(3x\right)^2-6^2=\left(3x-6\right)\left(3x+6\right)\)

7) \(\left(3x\right)^2-y^2=\left(3x+y\right)\left(3x-y\right)\)

8) \(x^2-\left(2y\right)^2=\left(x+2y\right)\left(x-2y\right)\)

9) \(\left(2x\right)^2-y^2=\left(2x-y\right)\left(2x+y\right)\)

\(3x-\dfrac{3}{5}=\dfrac{-7}{10}\\ 3x=\dfrac{-7}{10}+\dfrac{3}{5}\\ 3x=\dfrac{-7}{10}+\dfrac{6}{10}\\ 3x=\dfrac{-1}{10}\\ x=\dfrac{-1}{30}\)

___________________

\(\dfrac{2}{3}+\dfrac{1}{3}:x=\dfrac{3}{5}\\ \dfrac{1}{3}:x=\dfrac{3}{5}-\dfrac{2}{3}\\ \dfrac{1}{3}:x=\dfrac{9}{15}-\dfrac{10}{15}\\ \dfrac{1}{3}:x=\dfrac{-1}{15}\\ x=\dfrac{1}{3}:\dfrac{-1}{15}\\ x=-5\)

\(3x-\dfrac{3}{5}=-\dfrac{7}{10}\)

\(3x\)        \(=-\dfrac{7}{10}+\dfrac{3}{5}\)

\(3x\)        \(=-\dfrac{1}{10}\)

 \(x\)         \(=-\dfrac{1}{10}:3\)

 \(x\)         \(=-\dfrac{1}{30}\)

Vậy \(x=-\dfrac{1}{30}\)

\(\dfrac{2}{3}+\dfrac{1}{3}:x=\dfrac{3}{5}\)

        \(\dfrac{1}{3}:x=\dfrac{3}{5}-\dfrac{2}{3}\)

        \(\dfrac{1}{3}:x=-\dfrac{1}{15}\)

              \(x=\dfrac{1}{3}:-\dfrac{1}{15}\)

              \(x=-5\)

Vậy \(x=-5\)

\(\left(\dfrac{2}{7}-\dfrac{9}{4}\right)-\left(-\dfrac{3}{7}+\dfrac{5}{4}\right)-\left(\dfrac{2}{4}-\dfrac{9}{7}\right)\)

\(=\dfrac{2}{7}-\dfrac{9}{4}+\dfrac{3}{7}-\dfrac{5}{4}-\dfrac{2}{4}+\dfrac{9}{7}\)

\(=\left(\dfrac{2}{7}+\dfrac{3}{7}+\dfrac{9}{7}\right)-\left(\dfrac{9}{4}+\dfrac{5}{4}+\dfrac{2}{4}\right)\)

\(=2-4\)

\(=-2\)

-2

\(\dfrac{4}{7}+\dfrac{3}{7}x=\dfrac{1}{2}\\ \dfrac{3}{7}x=\dfrac{1}{2}-\dfrac{4}{7}\\ \dfrac{3}{7}x=-\dfrac{1}{14}\\ x=-\dfrac{1}{14}:\dfrac{3}{7}\\ x=-\dfrac{1}{6}\)

Vậy....

\(\dfrac{17}{6}-\left(x+\dfrac{7}{6}\right)=\dfrac{7}{4}\\x+\dfrac{7}{6}=\dfrac{17}{6}-\dfrac{7}{4}\\ x+\dfrac{7}{6}=\dfrac{13}{12}\\ x=\dfrac{13}{12}-\dfrac{7}{6}\\ x=-\dfrac{1}{12} \)

Vậy....

\(\dfrac{4}{7}+\dfrac{3}{7}x=\dfrac{1}{2}\\ \Rightarrow\dfrac{3}{7}x=\dfrac{1}{2}-\dfrac{4}{7}\\ \Rightarrow\dfrac{3}{7}x=-\dfrac{1}{14}\\ \Rightarrow x=-\dfrac{1}{14}:\dfrac{3}{7}\\ \Rightarrow x=-\dfrac{1}{6}\)

___________

\(\dfrac{17}{6}-\left(x+\dfrac{7}{6}\right)=\dfrac{7}{4}\\ \Rightarrow\dfrac{17}{6}-x-\dfrac{7}{6}=\dfrac{7}{4}\\ \Rightarrow\dfrac{5}{3}-x=\dfrac{7}{4}\\ \Rightarrow x=\dfrac{5}{3}-\dfrac{7}{4}\\ \Rightarrow x=-\dfrac{1}{12}\)

\(\left(\dfrac{1}{2}-\dfrac{1}{3}\right)-\left(\dfrac{5}{3}-\dfrac{3}{2}\right)+\left(\dfrac{7}{3}-\dfrac{5}{2}\right)\\ =\dfrac{1}{2}-\dfrac{1}{3}-\dfrac{5}{3}+\dfrac{3}{2}+\dfrac{7}{3}-\dfrac{5}{2}\\ =\left(\dfrac{1}{2}+\dfrac{3}{2}-\dfrac{5}{2}\right)+\left(-\dfrac{1}{3}-\dfrac{5}{3}+\dfrac{7}{3}\right)\\ =-\dfrac{1}{2}+\dfrac{1}{3}\\ =-\dfrac{1}{6}\)