Let's simplify the following fractions:
a) $\frac{(a+b)^{2}\left(a^{3}-b^{3}\right)}{\left(a^{2}-b^{2}\right)^{2}}$,
b) $\frac{6 a^{2} b^{2}-3 a^{3} b-3 a b^{3}}{a b^{3}-a^{3} b}$.
a) $\frac{a^{2}+a b+b^{2}}{a-b}$.
b) $3 \frac{a-b}{a+b}$.

Number of solutions: 25.