记 b = arctan(5/12),
12/13cosa-5/13sina
= (cosb)(cosa) - (sinb)(sina)
= cos(b + a) = 3/5
b + a = arccos(3/5) + 2kPI,或者
b + a = 2kPI - arccos(3/5).
所以,
a = arccos(3/5) - arctan(5/12) + 2kPI,或者
a = 2kPI - arccos(3/5) - arctan(5/12).
k = 0,1,-1,2,-2,...