设l1:y=kx+b
代入A,则kx-y-k=0
B到l1距离即为l1到l2距离
|-5-k|/√(k²+1)=5
12k²-5k=0
k=0或k=5/12
l1:y=0 l2:y=5
或l1:y=5x/12-5/12 l2:y=5x/12+5