Why negative value is not valid for log?

Answer 1

The logarithm function is the inverse of the exponential function. Take the exponential function (base 10): y = 10x. The inverse of this is x = 10y. The function y = log(x) is used to define this inverse function.

First look at y = 10x. Any real value of x will yield a positive real value for y. If x = 0, then y = 1; if x < 0 (negative) then y is between 0 and 1 (it will never equal zero, though). A value of 10-99999 is very close to zero, but not quite there. There are no real values of x which will give a negative y value for y = 10x.

Now look at y = log(x) or x = 10y. No matter what real values for y, that we choose, x will always be a positive number, so a negative value of x in y = log(x) is not possible if you are limiting to real numbers.

It is possible with complex and imaginary numbers to take a log of a negative number, or to get a negative answer to y = 10x.