The voltage gain of an amplifier with 9 \% negative feedback is 10....

The process of injecting a fraction of output energy of some device back to the input is known as feedback. When the feedback energy (voltage or current) is out of phase with the input signal and thus opposes it, it is called negative feedback. Voltage gain with feedback is $A_{v f}=\frac{A_\nu}{1+\beta A_\nu}$ where $A_\psi$ is voltage gain without feedback and $\beta$ is negative feedback. Given $A_{v f}=10, \beta=9 \%=\frac{9}{100}$$\begin{array}{ll} \therefore & 10=\frac{A_\nu}{1+\frac{9}{100} A_\nu} \\ \text{or } & 10+\frac{9}{10} A_\nu=A_\nu \\ \text{or } & 0.1 A_\nu=10 \text{ or } A_\nu=100 \end{array}$$$