Problem: 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 ?
Rationale: "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 r 3 + 1 = 9 r 3 = 8 r = 2 the answer is c"
annotated_formula: divide(multiply(20, 15), const_2)