1. ($$\frac{a}{b}-\frac{b}{a}$$) $$\cdot$$ $$\frac{3ab}{a+b}$$ = $$\frac{a^2-b^2}{ab}$$\cdot$$\frac{3ab}{a+b}$$ = $$\frac{(a-b)(a+b)3ab}{ab(a+b)}$$ = 3(a-b) = 3a-3b
Ответ: 3a-3b