А2. Упростите выражение: 15b 8b 7b+21 3-b+ b²-9+4.

$$ \frac{15b}{3-b} + \frac{8b}{b^2-9} + \frac{7b+21}{4} = \frac{-15b}{b-3} + \frac{8b}{(b-3)(b+3)} + \frac{7(b+3)}{4} = \frac{-15b \cdot 4(b+3) + 8b \cdot 4 + 7(b+3)(b-3)}{4(b-3)(b+3)} = \frac{-60b^2 - 180b + 32b + 7b^2 - 63}{4(b-3)(b+3)} = \frac{-53b^2 - 148b - 63}{4(b-3)(b+3)} $$

Ответ:$$ \frac{-53b^2 - 148b - 63}{4(b-3)(b+3)} $$