1. subtraction function. Mathematically speaking, because it is the negative power of e, and e is greater than 0, that is, exponential decay; Generally speaking, radioactivity will definitely decrease with time:)

2. Shift the term, and then take the same logarithm to get t=-ln(N/N0)/ in.

3.N=N0/2, that is, N/N0=0.5, from t=-ln(N/N0)/ in =0.693/ in. This 0.693/ in becomes the half-life, that is, the time required for radioactive decay to half.