Đề bài

Tính:

a) $\left( {a + 1 + \frac{{1 - 2{a^2}}}{{a - 1}}} \right):\left( {1 - \frac{1}{{1 - a}}} \right)$;

b) $\left( {\frac{a}{{{b^2}}} - \frac{1}{a}} \right):\left( {\frac{1}{b} + \frac{1}{a}} \right)$;

c) $\left( {a - \frac{{4ab}}{{a + b}} + b} \right).\left( {a + \frac{{4ab}}{{a - b}} - b} \right)$;

d) $ab + \frac{{ab}}{{a + b}}\left( {\frac{{a + b}}{{a - b}} - a - b} \right)$.