Concept:
For a transistor in common emitter configuration,
\[
\text{Power Gain}=\text{Current Gain}\times\text{Voltage Gain}
\]
If the current gain is $\beta$, then
\[
A_v=\beta\left(\frac{R_c}{R_b}\right)
\]
Therefore,
\[
A_p=\beta\times A_v
\]
\[
A_p=\beta^2\left(\frac{R_c}{R_b}\right)
\]
Step 1: Substitute the quantities given in the question
Given,
\[
A_p=A,\qquad R_b=B,\qquad R_c=C
\]
Hence,
\[
A=\beta^2\left(\frac{C}{B}\right)
\]
Step 2: Solve for $\beta$
\[
\beta^2=\frac{AB}{C}
\]
Taking square root,
\[
\beta=\sqrt{\frac{AB}{C}}
\]
Therefore,
\[
\boxed{\beta=\sqrt{\frac{AB}{C}}}
\]