The twentieth century was often called the Era of the Book. In those days, there were books about everything, from ant eaters to Zulus. Books taught people how , when , where , and why to everything. They illustrated, educated, punctuated, and even decorated
Assertion (A): The sum of the first fifteen terms of the AP $ 21, 18, 15, 12, \dots $ is zero.
Reason (R): The sum of the first $ n $ terms of an AP with first term $ a $ and common difference $ d $ is given by: $ S_n = \frac{n}{2} \left[ a + (n - 1) d \right]. $