AREA唔明

圖片參考：http://homelf.kimo.com.tw/o_ohm_o/yy.JPG

BD = √(a^2 + a^2)
BD = (√2)a

BF = BD = (√2)a

∠BCE = 45˚ (main point)

by sin law
BC / sin∠BFC = BF / sin45˚
a / sin∠BFC = (√2)a / (1/√2)
1/2 = sin∠BFC
∠BFC = 30˚

∠BDE = ∠BFC = 30˚

length of the perpendicular from B to DE
= (√2)a x sin30˚
= (√2)a / 2
= a / √2