Let the positive numbers a,b,c,d be in A.P. Then abc, abd, acd, bcd are [2001]
(4)
a,b,c,d are in A.P. ∴ d,c,b,a are also in A.P.
⇒dabcd, cabcd, babcd, aabcd are also in A.P.
⇒1abc, 1abd, 1acd, 1bcd are in A.P.
∴ abc, abd, acd, bcd are in H.P.