In this case it is OK to make the simplification, so we do. They will not contribute on average. So we grab this term. Where n is a non-negative integer. Let's isolate theta.
T is the period of orbit.
Thus Mercury's precessional rate in seconds of arc per century due to relativistic effects is 43.03. The observed value is 43.11 so that the prediction of General Relativity is confirmed in striking fashion.