$F = \frac{R^2 / k}{(1 - R^2) / (n - k - 1)}$

$F = \frac{R^2 / (n - k - 1)}{(1 - R^2) / k}$

$F = \frac{(1 - R^2) / k}{R^2 / (n - k - 1)}$

$F = R^2 \times (n - k - 1)$