3. Представьте выражение в виде дроби: a) $$\frac{28p^4}{q^6} \cdot \frac{q^5}{56p^4}$$; б) $$\frac{72x^3 y}{z} : (30x^2 y)$$; в) $$\frac{x^2-1}{x^2-9} \cdot \frac{5x+10}{x-1}$$; г) $$\frac{y+c}{c} \cdot (\frac{c}{y} + \frac{c}{y+c})$$

Решение: a) $$\frac{28p^4}{q^6} \cdot \frac{q^5}{56p^4} = \frac{28p^4 q^5}{56p^4 q^6} = \frac{1}{2q}$$ б) $$\frac{72x^3 y}{z} : (30x^2 y) = \frac{72x^3 y}{z} \cdot \frac{1}{30x^2 y} = \frac{72x^3 y}{30x^2 y z} = \frac{12x}{5z}$$ в) $$\frac{x^2-1}{x^2-9} \cdot \frac{5x+10}{x-1} = \frac{(x-1)(x+1)}{(x-3)(x+3)} \cdot \frac{5(x+2)}{x-1} = \frac{(x+1)5(x+2)}{(x-3)(x+3)} = \frac{5(x+1)(x+2)}{(x-3)(x+3)}$$ г) $$\frac{y+c}{c} \cdot (\frac{c}{y} + \frac{c}{y+c}) = \frac{y+c}{c} \cdot (\frac{c(y+c) + cy}{y(y+c)}) = \frac{y+c}{c} \cdot (\frac{cy + c^2 + cy}{y(y+c)}) = \frac{y+c}{c} \cdot (\frac{2cy + c^2}{y(y+c)}) = \frac{y+c}{c} \cdot \frac{c(2y + c)}{y(y+c)} = \frac{2y+c}{y}$$ Ответ: a) $$\frac{1}{2q}$$ б) $$\frac{12x}{5z}$$ в) $$\frac{5(x+1)(x+2)}{(x-3)(x+3)}$$ г) $$\frac{2y+c}{y}$$