Closed Loop Gain Approximation
I think it's high school math. Take \$\frac{1}{AB}=x\$. Then multiply the num. and denom. of the relation for \$\frac{Y}{X}\$ by x: You get \$\frac{Y}{X}=\frac{1}{B}.(1+x)^{-1}\$. Using binomial approximation and noting that x is much less than the unity, the approximation becomes \$ \frac{Y}{X} \approx\ \frac{1}{B}[1-x]\$ or \$ \frac{Y}{X}\ \approx\ \frac{1}{B}[1-\frac{1}{AB}]\$. Note that this approximation neglects second and higher order terms in the Taylor series of the function \$\frac{Y}{X}\$. \end