Odpowiedź i wyjaśnienie:
[tex]\dfrac{F}{2\frac{\pi d^2}{4}}\leq k_r\\\\\\\dfrac{F}{\frac{\pi d^2}{2}}\leq k_r\ /\cdot\dfrac{\pi d^2}{2}\\\\\\F\leq k_r\cdot\dfrac{\pi d^2}{2}\ /\cdot\dfrac{2}{k_r\cdot \pi}\\\\\\\dfrac{2F}{k_r\cdot \pi}\leq d^2\\\\\\d\geq\sqrt{\dfrac{2F}{k_r\cdot \pi}}[/tex]
