If 'a' ,'b', 'c' are in A.P in any OBJECTIVE EXAMINATION, we can assume 1,2,3 as 'a', 'b' ,'c'.

               Example: If a,b,c,d,e,f are in A.P, then (e-c) is equal to

                            (A) 2(c-a)     (B) 2(d-c)  (C) 2(f-d)  (D) d-c

                Solution: Let a,b,c,d,e,f as 1,2,3,4,5,6   then e-c= 5-3 =2

                        (i)option 2(c-a) =2(3-1) = 4   which is wrong.

                        (ii)option 2(d-c) = 2(4-3) =2  right.

                        (iii)option 2(f-d) =2(6-4) = 4  which is also wrong.

                         (iv)option (d-c) = 4-3 = 1  which is also wrong.

    Similarly, if we have to take three numbers in G.P, then we take 1,2,4 or 1,3,9.  

   Similarly, if we have to take three numbers in H.P, then we can take 2,3,6,  and for four numbers 3,4,6,12.

   Now this is another trick...If 'a', 'b','c' are three numbers given, then 

           1. if (a-b)/(b-c) = a/a ,  the no.s are in A.P,

           2. if(a-b)/(b-c) = a/b , the no.s are in G.P ,

           3. if (a-b)/(b-c) = a/c , the no.s are in H.P.

  Hope!! u all would find these tricks useful.