The sum of the fourth and twelfth term of an arithmetic progression is 20. What is the sum of the first 15 terms of the arithmetic progression? Choose the correct answer:
A) 3 B) 1/3 C) 2 D) 9 E) 1/9
The sum of the first n terms of a G.P. is given by , where 'a' is the first term of the G.P., 'r' is the common ratio and 'n' is the number of terms in the G.P.   Therefore, the sum of the first 6 terms of the G.P will be equal to   And sum of the first 3 terms of the G.P. will be equal to   Use the ratio between these two sums to find 'r'   The ratio of the sum of the first 6 terms : sum of first 3 terms = 9 : 1   i.e.   Or r3 + 1 = 9   r3 = 8   r = 2   The answer is 2
<result>C</result>
The sum of the fourth and twelfth term of an arithmetic progression is 20. What is the sum of the first 15 terms of the arithmetic progression?