al = 1/a = a 1 (l-1)d(1)
am = 1/b = a 1 (m-1)d ( 2)
an = 1/c = a 1 (n-1)d(3)
(1)-(2): (l-m)d = 1/a - 1/b, (l-m)ab=(b-a)/d (4).
(2)-(3)De: (m-n)d = 1/b-1/c, (m-n)bc=(c-b)/d (5)
( 3)-(1): (n-l)d = 1/c-1/a, (n-l)ca=(a-c)/d (6)
(4) (5) (6):
(l-m)a b (m-n)BC (n-l)ca =(b-a c-b a-c)/d = 0
.