(9) \( 8 a_{4}=2 a_{15}, a_{3}^{2}=2 a_{11} \) \( \begin{array}{ll}\text { c) } a_{3} \cdot a_{5}=-21, a_{5} \cdot a_{7}=39 & \text { b) } a_{3} \cdot a_{4}=77, a_{2}+a_{3}+a_{4}=21 \\ \text { e) } a_{2} \cdot a_{5}=-5, a_{3} \cdot a_{4}=3 & \text { d) } a_{1} \cdot a_{4}=58, a_{1}+a_{2}+a_{3}+a_{4}=62 \\ a_{1}^{2}+a_{8}^{2}=400, a_{7}+a_{8}+a_{10}=-4\end{array} \)

\( \frac{\frac{8}{5 x}-\frac{1}{x}}{\frac{4}{5}+\frac{3}{x}} \) \( \frac{\frac{8}{5 x}-\frac{1}{x}}{\frac{4}{5}+\frac{3}{x}}=\square( \) Simplify your answer. \( ) \)

a) \( \quad x^{2}=49 \)

Perform the indicated operation. \( \frac{8}{x^{2}-2 x-35}+\frac{7}{x-7} \) \( \frac{8}{x^{2}-2 x-35}+\frac{7}{x-7}=\square( \) Simplify your answer. \( ) \)

b) \( : \sqrt{x}=3 \)

Simplify the following expression. \[ \frac{x+8 y}{x-2 y} \div\left(\frac{1}{8 y}-\frac{1}{x+8 y}\right) \] \( \frac{x+8 y}{x-2 y} \div\left(\frac{1}{8 y}-\frac{1}{x+8 y}\right)=\square \) (Simplify your answer.)