Question Number 57390 by naka3546 last updated on 03/Apr/19
$${a}^{\mathrm{2}} \:+\:{d}^{\mathrm{2}} \:−\:{ad}\:\:=\:\:{b}^{\mathrm{2}} \:+\:{c}^{\mathrm{2}} \:+\:{bc} \\ $$$${a}^{\mathrm{2}} \:+\:{b}^{\mathrm{2}} \:\:=\:\:{c}^{\mathrm{2}} \:+\:{d}^{\mathrm{2}} \\ $$$$\frac{{ab}\:+\:{cd}}{{ad}\:+\:{bc}}\:\:=\:\:? \\ $$