BD=5, according to the right angle theorem of 3 strands, 4 strings and 5, it is easy to draw such a conclusion. Look at this picture, it may give you some inspiration.

I don't have time now. If I have time, I promise to explain it to you clearly. Besides, if you are willing to give me extra points, we can do it easily. If you don't pay a dime, hey! Hey! .....