math 2 15

x 2 + v 2 v S n n 1 n n1 1 n-1 rank u v 1 x + y x + y (a + b) (c + d) a b c d (a + b) a b (c + d) c d a b c d ( a + b ) ( c + d ) ab cd ( a + b ) ( c + d ) ab cd X 1 X n n n1 rank X u v 1