Ответ:

[tex]( \frac{1}{5} c - \frac{2}{13} d) \times ( \frac{1}{5} c + \frac{2}{13} d) =\left(\frac{1}{5}c\right)^{2}-\left(\frac{2}{13}d\right)^{2} = \left(\frac{1}{5}\right)^{2}c^{2}-\left(\frac{2}{13}d\right)^{2} =\frac{1}{25}c^{2}-\left(\frac{2}{13}d\right)^{2}= \frac{1}{25}c^{2}-\left(\frac{2}{13}\right)^{2}d^{2} =\frac{1}{25}c^{2}-\frac{4}{169}d^{2} [/tex]