Have you ever heard of complex number? In not complex number, have you ever heard that, in real numbers, square root of negative number cannot be found; in other words, the equation
x² = −1
has no solutions in read numbers?! And, of course, the same stands for 0.8 and −0.9.
This is something like middle school… and such school should be a prerequisite for starting with computing.
And NaN means "not a number", important object reserved to IEEE 754 floating number representation to represent the concept of invalid number. Please see:
http://en.wikipedia.org/wiki/IEEE_754[
^].
(Sigh…)
—SA