For symmetric data (e.g., a-2d, a-d, a, a+d, a+2d), the mean is the middle value (a), and variance simplifies because sum of deviations from mean has a pattern. For equally spaced data centered at a with spacing d and n terms: Var = $d^2$$\frac{n^2-1}{12}$. This is extremely useful for MCQ speed. Example: {3,5,7,9,11} → a=7, d=2, n=5, Var = 4$\frac{25-1}{12}$ = 4*24/12 = 8.