急 ! 數 Plane Geometry 問題1條 ?

As triangle ABE is an isosceles right-angled triangle,
with angle A = angle B = 45 degrees,
so, BE / AB = cos 45 = 1 / √2,
therefore,
AB / BE = 1 / cos 45 = √2

Also, triangle ABD is an isosceles right-angled triangle,
with angle B = angle D = 45 degrees,
so, AB / DB = cos 45 = 1 / √2 = √2 / 2

因為 角AEB=角BEC=角CED=角DEA=90