Triangle

Note that we have similar triangles, such that :

BDA ≈ BAC   ....so....

BA/DA  = BC/AC

And BC  = sqrt(10^2 + 20^2)  = sqrt(500)  = 10sqrt(5)    .......so....

10/DA  = 10sqrt(5) /20

10/DA  = sqrt(5) / 2    cross-multiply

20 = sqrt(5) * DA

DA = 20/sqrt(5)  = 20sqrt(5)  / 5   = 4sqrt(5)  →  (C)

I wanna help you, But I cannot see it :(

http://s43.radikal.ru/i101/1601/c3/84168694f4a9.jpg

Ughh. Blockeed. Can you messege me the picture? :)

enter from the link below may be it will show

Thanks CPhill !