The decomposition of a compound A follows first-order kinetics. The concentration of A at time t = 0 is 1.0 mol L-1. After 60 minutes, it reduces to 0.25 mol L-1. What is the initial rate of the reaction at t = 0? (Take ln 2 = 0.693)
If \( \sqrt{5} - i\sqrt{15} = r(\cos\theta + i\sin\theta), -\pi < \theta < \pi, \) then
\[ r^2(\sec\theta + 3\csc^2\theta) = \]
For \( n \in \mathbb{N} \), the largest positive integer that divides \( 81^n + 20n - 1 \) is \( k \). If \( S \) is the sum of all positive divisors of \( k \), then find \( S - k \).