Exact formula: g_h = GM/(R+h)^2 = gR^2/(R+h)^2

Approximation (h << R): g_h ≈ g(1 - 2h/R)

Derivation of approximation: g_h = g/(1+h/R)^2 ≈ g(1 - 2h/R) using binomial approximation (1+x)^n ≈ 1+nx for x << 1.

When to use which:

• If h/R > 0.1 (i.e., h > 640 km), use the exact formula
• If h << R, the approximation is sufficient

At height h = R: g_h = g/4 (not g/2! — inverse square, not inverse).