F.4 mathematics

20 = 2 x 2 x 5 = 10/5 x 10/5 x 5
so log 20 = log[10/5 x 10/5 x 5] = log 10/5 + log 10/5 + log 5
= log 10 - log 5 + log 10 - log 5 + log 5
= 2 log 10 - log 5
= 2 - a.

2012-01-08 08:33:36 補充：
Remark : Of course calculation could be much simpler if you immediately aware that 20 = 100/5, but this is not always the case.

log20
=log(4*5)
=log5+log4
=a+log4