Solution: Let the four terms be $ a - \frac3d2,\ a - \fracd2,\ a + \fracd2,\ a + \frac3d2 $. This ensures the terms are in arithmetic progression with common difference $ d $. The first term is $ a - \frac3d2 $, and the last term is $ a + \frac3d2 $. Their sum of squares is: - Nelissen Grade advocaten