Consider the arithmetic progression (A.P.) 7, 10, 13, …
a. (i) State the formula for the sum of the first n terms of an A.P.
b. (ii) Let Sp and Sq(q>p) denote the sums of the first p and first q terms of an A.P. Explain, without using specific numbers, why the sum of the terms from the (p+1)th term to the qth term is Sq−Sp.
c. (iii) Using the result of part (ii), find the sum of the terms from the 6th term to the 15th term (both inclusive) of the given A.P.