Why does #lna - lnb = ln(a/b)#?
It does not matter what base we use providing the same base is used for all logarithms, here we are using bease #e#.
Cerchiamo di definire #A,B.C# as follows=:
# A = ln a iff a = e^A #,
# B = ln b iff b = e^B #
# C = ln (a/b) iff a/b = e^C #
From the last definition we have:
# a/b = e^C => e^C = (e^A)/(e^B) #
And using the law of indices:
# e^C = (e^A) (e^-B) = e^(A-B) #
And as as the exponential is a #1:1# monotonic continuous function, we have:
# C = A-B #
E così:
# ln (a/b) = ln a - ln b # QED