General Term of an AP :-
   an = a + (n-1)d
   where a = first term of an AP,
   d = common difference of an AP,
   n = no. of terms an AP. 
The sum of the first n terms of an AP is :-
   Sn = n/2[2a + (n-1)d]
If l is the last term of the finite AP, say the nth term, then the sum of all terms of the AP is :-
   Sn = n/2[a + l]
If a, b, c are in AP, then b is called Arithmetic Mean of a and c.
   b = (a + c)/2

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