Exercise 10

By solving the following system of equations:

\[ \begin{cases} \cfrac{2}{x} - \cfrac{1}{y} = 2 \\[.5em] \cfrac{1}{x} + \cfrac{2}{y} = 11 \end{cases} \]

we obtain that \(\frac{x}{y}\) equals to:

  1. \(\cfrac{4}{3}\)

  2. \(\cfrac{3}{4}\)

  3. \(\cfrac{1}{12}\)

  4. \(-\cfrac{16}{5}\)

  5. \(-\cfrac{26}{23}\)

Try to solve it before checking the answer.
  1. \(\cfrac{4}{3}\)

Multiplying both equations by \(x\):

\(
2 – \cfrac{x}{y} = 2x
\hspace{4em} \boldsymbol{(2)}
\)

\(
1 + 2 \cfrac{x}{y} = 11x
\hspace{4em} \boldsymbol{(3)}
\)

Multiplying the equation (2) by -11, and the equation (3) by 2:

\(
-22 + 11 \cfrac{x}{y} = -22x
\hspace{4em} \boldsymbol{(4)}
\)

\(
2 + 4 \cfrac{x}{y} = 22x
\hspace{4em} \boldsymbol{(5)}
\)

Adding the above equations:

\[
\begin{aligned}
-20 + 15 \frac{x}{y} = 0
&\Rightarrow
15 \frac{x}{y} = 20
\\[.5em]
&\Rightarrow
\frac{x}{y} = \frac{20}{15}
\\[.5em]
&\Rightarrow
\frac{x}{y} = \boldsymbol{\frac{4}{3}}
\end{aligned}
\]