If are in A.P., then prove that
are in A.P.
will be in A.P.
if will be in A.P. (By adding 1 to each number)
if will be in A.P.
if will be in A.P. (By dividing each number a + b + c)
if if
if if (b - a) (b + a) = (c - b) (c + b)
if
if are in A.P. which is true.
Hence, are in A.P.