Why is there Newton is Einstein's field equation?

Question

If Newton was not exact and needed refinement in the gravity equation how did he fit in the Einstein Field Equation?

Answer 1

If you do a small velocity, low density approximation in Einstein's equation, you get Newton's equation for gravity. This is demonstrated in most General Relativity texts. In the same way, Quantum mechanics reduced to Newtonian physics when the expected values for position, etc. are used.