Using good quality seeds of improved varieties significantly enhances crop yield by ensuring better germination, resistance to pests, and tolerance to environmental conditions. On average, such seeds improve yield by 20-25%.
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: $S_n = \frac{n}{2} \left[ a + (n - 1) d \right].$