The first and the last terms of an AP are 7 and 49 respectively. If sum of all its terms is 420, find its common difference.
Let the common difference, first term and last term of the AP be d, a and l, respectively.
Suppose the number of terms in the given AP be n.
Sum of n terms of an AP, Sn=n/2 (a+l)
a = 7
l = 49
Thus, the common difference of the A.P. is 3.