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