Ответ:

[tex]1)\ \frac{7c}{c^2+ac}\\\\2)\ \frac{7x-7y}{x^2-2xy+y^2}\\\\3)\ \frac{b+7}{b^2-49}\\\\[/tex]

Объяснение:

[tex]1)\\\\\frac{7}{c+a}=\frac{7\cdot c}{c(c+a)}=\frac{7c}{c^2+ac}\\\\2)\\\\\frac{a}{x-y}=\frac{7\cdot (x-y)}{(x-y)(x-y)}=\frac{7x-7y}{x^2-2xy+y^2}\\\\3)\\\\\frac{1}{b-7}=\frac{1\cdot(b+7)}{(b-7)(b+7)}=\frac{b+7}{b^2-49}\\\\[/tex]