Ta có:  x + y = x : y

   =>  x = x .y + y = y ( x+ 1 )   (1)

   => x : y = y (x + 1) : y = x - 1

Do đó, ta có: \(\begin{cases}x:y=x+1\\x:y=x-y\end{cases}\)

=> x - 1 = x + y

=>       -1 = y

=>       y = -1

Thay -1 vào (1) ta được:

   x = -1(x+1)

 => x = -x . -1

=> 2x = -1

=>   x = \(\frac{1}{2}\)

Vậy x = \(\frac{1}{2}\)

       y = 1

A={5;7;9}

A={x  ϵ N/ 2x + 1; 3 < x < 10}

\(\frac{\frac{2}{11}-\frac{2}{13}+\frac{2}{17}}{\frac{5}{11}-\frac{5}{13}+\frac{5}{17}}\)

\(=\frac{2\left(\frac{1}{11}-\frac{1}{13}+\frac{1}{17}\right)}{5\left(\frac{1}{11}-\frac{1}{13}+\frac{1}{17}\right)}\)

=\(\frac{2}{5}\)

competitions
After three years more than the 27 year old mother still. Through my remarks on this problem in the form of know-performance-ratio estimates.
I have diagrams: (Age child and maternal age 3 years)
  Mother: l l ----- ----- ----- l l l -----
Con: l ----- l 27
Effect of equal parts of:
4-1 = 3 (part)
Three years after her mother's age:
27: 3 x 4 = 36 (years old)
Mother's age today:
36-3 = 33 (years old)
Children age three years are:
36-27 = 9 (years)
The current age is:
9-3 = 6 (age)
Answer: Mom: 33
                Con: 6