log (X/Y) = log X - log Y

log XY = log X + log Y

log 4 = log ( 2的"2"次方) = 2x log2
log 8 = log ( 2的"3"次方) = 3x log2

log 50 = log (5x10) = log 5 + 1 = log (10/2) + 1 = (1-0.3010) +1
或是 log50 = log (100/2) = log 100 - log 2
= 1.6990

log 2=0.3010
log 3=0.4771
log 7=0.8451

背這三個就夠了 其他都用算的才不用背那麼多