Question Number 185384 by Mingma last updated on 20/Jan/23
Answered by HeferH last updated on 21/Jan/23
Commented by HeferH last updated on 21/Jan/23
$${R}\:=\:\frac{\mathrm{13}\:+\:\mathrm{2}{r}}{\mathrm{2}} \\ $$$$\left(\:\frac{\mathrm{13}+\mathrm{2}{r}}{\mathrm{2}}\:−\:{r}\right)^{\mathrm{2}} =\:{r}^{\mathrm{2}} \:+\:\left(\frac{\mathrm{13}\:+\:\mathrm{2}{r}}{\mathrm{2}}\:−\:{r}\:−\mathrm{4}\right)^{\mathrm{2}} \\ $$$$\:{r}\:=\:\mathrm{6}\: \\ $$$$\:\Rightarrow\:{R}\:=\:\frac{\mathrm{13}+\mathrm{12}}{\mathrm{2}}\:=\:\mathrm{12}.\mathrm{5}\: \\ $$