log幾時可以both sides take log?

log幾時可以both sides take log?
當2邊式都>0 ge時候可以take log
唔信你用計數機...計log 0/log負數...
會出現maths error..

呢條得唔得,可唔可以both side take log?
(2)[2^x]^2-2^x=10
你所講呢條式係唔可以...
因為...lhs:[2^x]^2-2^x>=0...
因為佢會講x會否唔等於0...
if x=0..咁ge話..lhs=0...log唔到..
但如果佢話明..x唔等於0
就可以take log

2008-06-06 19:43:33 補充：
因為佢會講x會否唔等於0...
係---因為佢無講x會否唔等於0...

而且x我地唔知佢會唔會=0..所以唔應該用log計..

Let 2^x = y, we get
2y^2 - y = 10
2y^2 - y - 10 = 0
(2y - 5)(y + 2) = 0
therefore, y = 5/2 or -1, that is 2^x = 2.5 or -1(reject because 2^x is always positive for all values of x).
therefore, xlog2 = log 2.5
therefore x = log (2.5)/log 2

2008-05-30 19:59:33 補充：
x=log(2.5)/log2 = 0.398/0.301 = 1.322