# Maximum power

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In P(max) formula how they got 4R

and after that how they got R(ab)

$R_{th}$ is the Thevenin equivalent resistance of the circuit measured across ab. For this you short circuit the battery.
Maximum power is transferred to the load across ab when this load is $R_{ab}=R_{th}$.
The power dissipated in $R_{ab}$ is $\frac{V_{ab}^2}{R_{ab}}$. I think the factor of $4$ must be a mistake.