

Given, a, b, c are in HP
1/b = 1/a + 1/c
=> 1/b = (a + c)/ac
=> ac = b(a + c)
=> ac = ab + bc
=> ac - ab - bc = 0
=> ac - ab - bc + b2 = b2 {adding b2 on both side}
=> a(c - b) - b(c - b) = b2
=> (a - b)*(c - b) = b2
=> (a - b)/2 *(c - b)/2 = b2 /4
=> (a - b)/2 *(c - b)/2 = (b/2)2
=> (a - b)/2 , b/2 , (c - b)/2 are in GP
