3. Упростите выражение: 3) $$\frac{36}{a^2+3a} - \frac{12}{a}$$; 4) $$\frac{20x^2}{4x-7} - 5x$$

3) $$\frac{36}{a^2+3a} - \frac{12}{a} = \frac{36}{a(a+3)} - \frac{12(a+3)}{a(a+3)} = \frac{36 - 12a - 36}{a(a+3)} = \frac{-12a}{a(a+3)} = \frac{-12}{a+3}$$
4) $$\frac{20x^2}{4x-7} - 5x = \frac{20x^2 - 5x(4x-7)}{4x-7} = \frac{20x^2 - 20x^2 + 35x}{4x-7} = \frac{35x}{4x-7}$$