Giải thích các bước giải:
\[\begin{array}{l}
a,\\
\frac{{\frac{3}{7} - \frac{3}{{11}} + \frac{3}{{13}}}}{{\frac{5}{7} - \frac{5}{{11}} + \frac{5}{{13}}}} + \frac{{\frac{1}{2} - \frac{1}{3} + \frac{1}{4}}}{{\frac{5}{4} - \frac{5}{6} + \frac{5}{8}}} = \frac{{3\left( {\frac{1}{7} - \frac{1}{{11}} + \frac{1}{{13}}} \right)}}{{5\left( {\frac{1}{7} - \frac{1}{{11}} + \frac{1}{{13}}} \right)}} + \frac{{\frac{1}{2} - \frac{1}{3} + \frac{1}{4}}}{{\frac{5}{2}\left( {\frac{1}{2} - \frac{1}{3} + \frac{1}{4}} \right)}} = \frac{3}{5} + \frac{2}{5} = 1
\end{array}\]
b,
Áp dụng dãy tỉ số bằng nhau bằng nhau ta có:
\[\begin{array}{l}
\frac{{y + z - x}}{x} = \frac{{z + x - y}}{y} = \frac{{x + y - z}}{z} = \frac{{\left( {y + z - x} \right) + \left( {z + x - y} \right) + \left( {x + y - z} \right)}}{{x + y + z}} = \frac{{x + y + z}}{{x + y + z}} = 1\\
\Leftrightarrow \left\{ \begin{array}{l}
y + z - x = x\\
z + x - y = y\\
x + y - z = z
\end{array} \right. \Rightarrow x = y = z
\end{array}\]