Make \( R \) the subject of the formular \[ \frac{1}{f}=k \frac{Q_{1} Q_{2}}{R^{2}} \]

To make \( R \) the subject of the formula, we start with: \[ \frac{1}{f} = k \frac{Q_{1} Q_{2}}{R^{2}} \] First, we'll multiply both sides by \( R^2 \): \[ R^2 \cdot \frac{1}{f} = k Q_{1} Q_{2} \] Next, we can rewrite it as: \[ R^2 = \frac{k Q_{1} Q_{2}}{\frac{1}{f}} = k Q_{1} Q_{2} f \] Now, take the square root of both sides to solve for \( R \): \[ R = \sqrt{k Q_{1} Q_{2} f} \] So the final expression with \( R \) as the subject is: \[ R = \sqrt{k Q_{1} Q_{2} f} \]