Biogas offers multiple benefits for rural India:
1. Clean cooking fuel: Reduces dependency on firewood, minimizing indoor pollution.
2. Energy source: Provides electricity and fuel for small-scale applications.
3. Waste management: Utilizes agricultural and animal waste, reducing environmental pollution.
4. Bio-fertilizer: The slurry left after biogas production is an excellent organic fertilizer.
5. Cost-effective: Reduces energy costs and promotes self-reliance.
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]. $