jimjohn wrote:

"If BD is the height of a right angled triangle ABC, the right angle has to be at vertex B."

just wondering how come this is so. why couldnt B be at the top of the triangle instead of at the right angle.

An assumption that i did not know i had made till ravshonbek pointed it out was that A & D are distinct points. If A & D are distinct points, you will find that if BD is the height of a right angled triangle, the right angle will always be at point B. If it were at any other point ( say A or C ), then BA or BC would be the height and BD would have to coincide with one of them.