I've compared my computer with my friend's accepted program.
I found that his main algorithm is to find the maximal viewing angle between (x1,0) and (x2,0) using binary search.

But I have a random data like this
"-41 328 150 467"
my program output 0.528878 {which meet the peak value at (173.423285,0) on the right side of (x2,0)}
but his program output 0.527316 {which meet the peak value at (150,0)}

Must the programmer view the Billboard bewteen (x1,0) and (x2,0)?

If I misunderstood the description of this problem,please tell me the right one.

Thank you very much.

my AC program returns 0.528878.

Thank you very much!
I will check my solution more carefully.