Summing this formula one obtains ( d x 1 ( u ) ) 2 + ⋯ ⋯ + ( d x n ( u ) ) 2 = 4 R 2 ( R 2 − − | u | 2 ) 2 [ ( d u 1 ) 2 + ⋯ ⋯ + ( d u n ) 2 ] + 16 R 4 ( R 2 − − | u | 2 ) ( u ⋅ ⋅ d u ) ( u ⋅ ⋅ d u ) + 16 R 4 | u | 2 ( u ⋅ ⋅ d u ) 2 ( R 2 − − | u | 2 ) 4 = 4 R 2 ( R 2 − − | u | 2 ) 2 [ ( d u 1 ) 2 + ⋯ ⋯ + ( d u n ) 2 ] ( R 2 − − | u | 2 ) 4 + R 2 16 R 4 ( u ⋅ ⋅ d u ) ( R 2 − − | u | 2 ) 4 .