The half-life of a radioactive substance is 10 days. After 30 days, the fraction remaining undecayed is: MCQ with Answer and Explanation

Explanation:
s_nth = u + a/2 (2n-1). u=0, so distances proportional to (2n-1): 1,3,5.

Explanation:
For a bipolar junction transistor (BJT) in common-emitter configuration, DC current gain β = I_C / I_B, where I_C is collector current, I_B base current. Typical β values range 20-200. Option D is α (common-base gain). Memory tip: 'β = collector current / base current; large β means small base current controls large collector current'. This definition-based question tests electronics fundamentals, frequently examined in competitive exams. Always distinguish β (CE configuration) from α (CB configuration) and their relationship: β = α/(1-α).

Explanation:
T = 2u sinθ/g, max when sinθ=1 => θ=90° (vertical).