Find the apsidal angle for a nearly circular orbit of

Question

A particle of mass m moves under the central field

F = -m (γe^-∈r/a/r²)r^{^},

where γ , a and ∈ are positive constants. Find the apsidal angle for a nearly circular orbit of radius a. When ∈ is small, show that the perihelion of the orbit advances by approximately π∈ on each revolution.