А2. Упростите выражение: 15b 8b 7b+21 3-b 62-9 4

$$\frac{15b}{3-b}+\frac{8b}{b^2-9}+\frac{7b+21}{4}=\frac{15b}{3-b}+\frac{8b}{(b-3)(b+3)}+\frac{7b+21}{4}=\frac{-15b}{b-3}+\frac{8b}{(b-3)(b+3)}+\frac{7(b+3)}{4}=\frac{-15b \cdot 4(b+3) + 8b \cdot 4 + 7(b+3) \cdot (b-3)4}{4(b-3)(b+3)}=\frac{-60b(b+3)+32b+28(b+3)(b-3)}{4(b-3)(b+3)}=\frac{-60b^2-180b+32b+28(b^2-9)}{4(b-3)(b+3)}=\frac{-60b^2-148b+28b^2-252}{4(b-3)(b+3)}=\frac{-32b^2-148b-252}{4(b-3)(b+3)}=\frac{-8b^2-37b-63}{(b-3)(b+3)}$$

Ответ: $$\frac{-8b^2-37b-63}{(b-3)(b+3)}$$