An acceleration of an orbiting body, in a central force, is defined as:

a(r,v) = -k/r^2(1 + v^2/c^2)

k = GM = const, v = dr/dt = r'

what is the precession of perihelion,
or the apsidal angle in this case?

[[Mod. note -- This appears to be a homework exercise; our usual
policy in the newsgroup is to not provide an answer but rather to
try to explain appropriate methods for the student to obtain the
answer themself.
-- jt]]