If a = 925, b = 924, c = 923, then

find 3(a² + b² + c² - ab - bc - ca).

9

a² + b² + c² - ab - bc - ca = \(\frac{1}{2}\) [(a - b)² + (b - c)² + (c - a)²]

                                       = \(\frac{1}{2}\) [(925 - 924)² + (924 - 923)² + (923 - 925)²]

                                       = \(\frac{1}{2}\) [1 + 1 + 4] = 3

                                       = 3 × 3 = 9