Bài giải

$\begin{align}& \frac{1}{15}+\frac{1}{35}+\frac{1}{63} \\ & =\frac{1}{3\times 5}+\frac{1}{5\times 7}+\frac{1}{7\times 9} \\ & =\frac{1}{2}\times \left( \frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9} \right) \\ & =\frac{1}{2}\times \left( \frac{1}{3}-\frac{1}{9} \right) \\ & =\frac{1}{2}\times \frac{2}{9} \\& =\frac{1}{9} \\ \end{align}$

Suy ra:

$\begin{align}& \frac{1}{9}\times x=1 \\ & x=1:\frac{1}{9}=9 \\ &  \\\end{align}$