Logs are defined only for positive numbers so the log of a negative number does not exist.
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Yes. The logarithm of 1 is zero; the logarithm of any number less than one is negative. For example, in base 10, log(0.1) = -1, log(0.01) = -2, log(0.001) = -3, etc.
No. The range of the exponential (antilog) function is the positive reals (unless you are dealing with the complex field).
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.
In order to find the log with a power of ten, use the LOG button. For example, to find log105, type log(5). (The parenthesis after the g will appear when you press the LOG button. In order to find a log with a power other than ten, you will have to divide by the log10 of that power. For example, to find log82, type log(8)/log(2). In order to find the natural log of a number, use the LN key. For example, to find the natural log of 91, type ln(91).
There is no answer - it is an error: negative numbers do not have logarithms. The log if a number tells to what power the (positive) base must be raised to get the number. Raising any positive number to any power will never result in a negative number, so it is an error to try and take the log of a negative number.