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
