Ответ:

Объяснение:

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[tex]\displaystyle\bf\\\bigg(\frac{1}{7} c-\frac{4}{5} d\bigg)\cdot\bigg(\frac{1}{7} c+\frac{4}{5} d\bigg)=\bigg(\frac{1}{7} c\bigg)^{2} -\bigg(\frac{4}{5} d\bigg)^{2} =\frac{1}{49} c^{2} -\frac{16}{25} d^{2}[/tex]