It is often said that we cannot take logarithm of a negative number. But wait! Suppose we can use complex numbers.
Euler formula
We begin with the Euler formula:
and put q = p, we get :
Therefore,
ln (– 1) = ip
If a > 0, then
ln (– a) = ln [a(– 1)] = ln a + ln (– 1) = ln a + ip
and
Logarithm of a complex number
By putting q = p/2 in the Euler formula, we get
Therefore
If z = b i, which is a purely imaginary number,
then
Finally, for the polar form of a complex number,
,
we get
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